If you are going to take this stuff seriously, you really need one (A good stick, custom if you are rich or clever enough). The 360's analogue is fine for inputs being so nice and smooth, but the buttons on a pad, any pad, just do no cut it for the combos you need to pull off in anything other than casual play.
This is just one of my current excuses for my sub 30% ranked wins.
Also, wait off on the fightpad as well, lots of complaints out there.
Looks like there is money to be made by someone who can put out a decent fighting game controller.