hes doesnt suck, just in comparison to what he could have been hes very weak. he could take out a fair few i think, in republic 78 he took out the dark woman. and luke kind of overwhelmed him in rotj, alot of rage, plus he appeared out of nowhere. vader never really got his bearings, i think luke would have won in the end anyway but with a bit of difficulty.
im reading rise of darth vader now so my guess is he will take a few out in it, and after reading crimson empire id say hes still very skilled. he toyed with the best of the red guard before killing him, and their pretty much as skilled as humans can be.
he doesn't suck, His construction does and i'm guessing he managed to tweak it enough to make it workable, hence all the suit design changes in the OT. He had to develope an entirely new style of combat for himself to due an awkward grip cause by his huge new hands and him now having to protect his center more, like Grievous had to.