How good is Envenomed Dagger?

I did a little searching and I've seen some mention of it. My main desire would be for AVs when the team doesn't have someone with a -Regen power.

I know it'll have a problem of missing since it can't be slotted for accuracy, but besides that how effective is it against AVs? I think it's listed as -250% regen but how heavily resisted is that? Will it make a difference if I'm the only character using it, or does most/all of the team have to use it in an AV fight for it to be noticeably effective?

A level 50 AV has, I believe, 85% resistance to regen debuffs, so the -250% becomes -37.5%. A level 50 AV also regenerates a little over 100 hp/sec. Since Envenomed Dagger lasts 10 seconds, that's more than 375 hp not regenerated, which is just as good as dealing that much damage. Add the 160 damage done by the power itself, and you have an attack that does almost 500 damage with a 1.00s animation time. If you have 3 people keeping the debuff up, you can negate the AV's regen completely, although more debuffs beyond that won't do anything.

If you use a weapon set, the redraw will make using Envenomed Dagger less efficient, but probably still better than using one of your normal attacks. I don't know whether or not the debuff will stack from the same caster (probably not), so using it more than once every 10 seconds is unnecessary.

Thanks Hopeling, very comprehensive and helpful answer!

I used it to solo my first AV before I got him slotted out. It halps a lot more than it looks like, and is one of my favorite purchasable temps.

I've used it once (which is 20 daggers in 'one' power right?). I found it very helpful to solo EBs/AVs on my Claws/SR brute (low 40s when I used the daggers). I was surprised at how much toxic damage it does to the AVs/EBs (hundreds of damage and not a piddly -1 or -2 dmg ticks). It's really helpful when you know that the EB/AV is about to hit their "godmode" power (especially someone like Valkyrie) to lessen their annoyance factor.

I'd say the best of the new temp powers would be:

Recovery Serum (all around useful and not just for a certain mob type)
Envenomed Dagger
Hand Grenades (again pretty surprised at how good the damage is (on my brute that is))

The ally rez is nice too, a couple of my non-melee characters carry that around so that I can pop a fellow squishy back up with no wakie stun if they go down. Nothing game-breaking, but useful.

Hopeling's analysis seems pretty much correct. As a supporting anecdote, a team of 4 (I was using the Dagger, I don't know if anyone else was) managed to take down a Paladin. It's a bit more than 20 daggers- 24 or 30- and I went through all of them. I know Paladin is not spectacular by GM standards but I still feel it was a good job and the dagger made it possible.

I really like the new temp powers. Especially recovery serum and the allied rez. The rez is so handy on a TF.