Well, I just got my kat/regen to 41. He's relatively effective. If you go hunting you'll find lots of comments by me on katana/regen, someone started a thread about a while back.
If you like thudding great damage numbers, dump Katana and go for Broadsword. Slightly slower, but heavier damage.
In either case, get Divine Avalanch/Parry early and make a lot of use of it. It's great.
Disclaimer: The above may be humerous, or at least may be an attempt at humour. Try reading it that way.
Posts are OOC unless noted to be IC, or in an IC thread.
How I have my katana attacks worked out:
Sting of the Wasp 3 damage 1 accuracy 2 recharge
Divine Avalanche 3 defence buff 1 accuracy 2 recharge
Soaring Dragon 3 damage 1 accuracy 2 recharge
Golden Dragonfly 2 damage 1 dmg/acc Hami-O 3 recharge (would be same as SD and SotW if it didn't have a hami-O)
You chain the three actuall attack attacks (ie. not DA) and when you get a slight gap that happens occasionaly you use DA. If you need alot of defence, you can keep DA stacked very easily.
Note: I'm not sure if this will be good on a non */SR, the attack chain is precariously balanced with so few attacks (I'd reccomend waiting to get to level 35 before ou even consider this build) and without Qickness you might not cut it.
I was thinking SR, as I've done the Regen before, but rather liked it so stuck with it. At the moment I'm more into accuracy than damage, and quick little attacks, so I'm thinking Katana goes well for me.
Shame I can't play at this point in time as (I'm assuming) my new RAM upgrade isn't compatible with my older one, and therefore I keep crashing.
level 41 katana/regen here too, feeling strong and happy with my build at the moment (have in fact felt strong and happy ever since I got Integration at 16, despite the nerfing that has been going on since). It's extremely strong soloing and in small teams and does well in larger teams too, provided I don't have to take the alpha strike. I went with the Fitness pool after my i6 respec and have been extremely happy with that choice.
I have all the attacks except Gambler's Cut, slotted 1acc, 3dam and 1-2 recharge. I can attack constantly without having to use all my attacks in the chain, sometimes DA is not needed, sometimes Lotus Drops would be a waste.
So here's what I think about the attacks. I have never had Gambler's Cut so I can't really comment on it, but I understand it does similar damage to DA but quicker. I had the single target taunt (Calling the Wolf) for a while and it had really stylish animation but not that much use because the range was so short.
Sting of the Wasp - the vanilla attack that's been in constant use. Using it a bit less now that I've got the two big hitters but it's still got a firm place in my single target chain
Flashing Steel - a nifty melee cone that does only slightly less damage than SotW. Take it, slot it
Build Up - 3-slotted this but slotting can wait. most useful on boss fights and with the AoE if you're taking that.
Divine Avalanche - a great parry move that stacks! Definitely recommended as it really makes a huge difference and goes well with Regen. I've slotted mine 2acc, 3defbuff, 1 recharge. The damage is tiny but you're not taking it for damage anyway. it works pretty well with just 1 acc, so it's a good idea to take it early IMO.
The Lotus Drops - this is your melee PbAoE. Some people don't take it but I've personally found it lots of fun and well worth it. The damage is a bit more than SotW, It does one bit hit and 3 ticks of DoT per target. Unlike some other sword circles, the activation time is not too bad.
Soaring Dragon - now we're really talking about an attack. This is great. It does pretty reliable knockup and great damage. The one downside is that you can't hit the foes when they're in the air, but this gets them out of the fight for a looong time
Golden Dragonfly - even nicer! Increased chance to crit, pretty reliable knockdown and great, great damage. The nice thing about katana's big hitters is that the recharge and end cost isn't that bad at all, so you can use them all the time. It's a very narrow cone but getting the mobs queued up for it is pretty hard so I hardly ever think about it. It's always a great surprise when it does hit multiple opponents. 
And my only annoyance with this was just fixed in the latest patch. It used to not connect to moving or knocked-down targets so I couldn't use it on runners or immediately after SD.
All in all, kat/regen is a great choice, very strong if not all that versatile as there are no ranged attacks or debuffs or even that many AoEs. All katana attacks are supposed to have a def debuff, but I can't say it feels that noticeable.
Divine Avalanche adds some versatility and greatly increased survivability. Stack it and you can play a mini-tank sometimes.
Hope this helps some. Katana FTW!
Edit: uh oh, Statesman is posting here as we speak. What I meant to say, obviously, is that Kat/Regen is totally gimped, I can't solo and feel useless on teams. 
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I was thinking SR, as I've done the Regen before, but rather liked it so stuck with it.
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SR is a love or hate set - you have to have the mindset that sometimes you will be a God and nothing will hit you, and sometimes you'll be faceplanted by a -1 mob and accept both possibilities as the blessing and curse of the set
SR is the set for true suicide scrappers
Go Claws! Not as much damage but soooo much cooler and more fun!
After being DM/Regen, I would suggest going the other way with it all and going Spines/DA.
You go from single target monster to AoE monster.
World of Jackcraft.
So many AoEs that I don't think a Stalker would look at me twice...
I'm not all that much into Cone attacks and AoEs, I usually feel more comfortable with single target and PBAoE attacks, so I don't think I'll be getting Flashing Steel.
It's a melee cone, doesn't cost that much and recharges pretty quickly. You can use it like single target attack, only it hits the mobs beside your target as well. I wouldn't leave it out, Lotus Drops is a lot more situational.
Edit: also your damage on levels 1-25 will be extremely lacking
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It's a melee cone, doesn't cost that much and recharges pretty quickly. You can use it like single target attack, only it hits the mobs beside your target as well. I wouldn't leave it out, Lotus Drops is a lot more situational.
Edit: also your damage on levels 1-25 will be extremely lacking
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My eyes...opened! Simple and effective! I thank you for that vague woohooness!
#Honestly#
Hey, if you want to gimp yourself for 24 levels, go ahead and leave it out 
It's just that I don't much like cone blasts either because I'm too lazy to position myself before using them. Flashing Steel is a totally different story.
Got ya, Flashing Steel at 12...as that is a spot missing something to fit in(I was gonna get Hurdle but at the moment I cannot lose endurance even if I go against clocks... And I haven't SK'ed for Sappers yet ). I'm gonna get hurdle sometime, and 3 slot it, just for the fun!
Super Jumpy Jumpiness!
EDIT: Nearly forgot the swordiness too!
Mk..pretty pointless post here, but...I'm going back to my Scrappery self after many attempts at other ATs, and pretty much drastically failing.
My original is a Dark Melee/Regeneration. Now I've started a Katana/Regeneration, but I'm unsure if that is the best choice for me. Before I get too far on this, is there anyone with a different view, or any hints on the attacks of the Katana?