In my opinion Grav is a set you must have actively played before you can understand it; That may be one of the reasons it's not alll to popular.
I myself didn't think much of it because I prefer the regular AoE control to just lock enemies down, jump in and crush em to pieces (I'm actually playing a Dom, not a Controller, so you might want to take that into consideration).
The lower levels are kinda hard, since you're gonna have to juggle your foes to keep em occupied. That begins to change when you get GDF, which is by no means the ultimate solution, but it's a smooth transition to the rest coming later.
Then at 26 comes Wormhole - Take it, love it: You can teleport and at the same time stun a spawn; Anyway, it's hard to use in a team, since (at least in my experience) people don't understand it and then are considerably annoyed, so when I'm on a team I end up just using my secondaries and occasionally throwing in a GDF. However, the solo tactic might actually work well if they grant you moblead - I have yet to find a team that is willing to wait for me to get the mob instead of simply running towards it. Anyway, if you set the wormhole to open into a corner, you can REALLY clump the baddies together and tie them into a neat little package with your AoE Immob (which is the only one without knockback protection, hail the gods of gravity control!), so that you maximize whatever AoE damage you might get into the team (as a Dom, that would be me ^^)
Then Singy: I originally got Grav because I wanted to stack single target holds (due to Singy). I happen to be 32 right at the moment and noticed this: You don't really SEE an impact when Singy comes into play (unless you use the repel tactics mentioned in posts before) because the damage is negligible and well, the graphic effects of grav aren't exactly booms, but what you will most definitely notice is an increase in your survivability. Everything just runs smoother because Singy spams controls at a rate you would never have dared dream of.
That being said, as already mentioned by someone before, it's a good set for solo or small teams but gets run over by larger teams. However, it's really fun to play and a challenge everyone should have a taste at before judging the set, because honestly the mere power descriptions don't sound like it's gonna do anything at all (at least to me), but oh the fun!
If it is unlikely for Wormhole to be moved to level 12, I think the simplest changes that would vastly improve Gravity's performance would be to increase Wormhole's radius of effect and decrease its animation time.