i have a DSG and i had one of the older tiptronics before that. i think both those gearboxes were fabulous, but they both have their vices.
in regards to the DSG, it's a brilliant gearbox but that statement is engine specific. i've driven a few cars with it (A3 2.0 TDI 170, Golf V GTI, Golf V GTI Ed30, TT 3.2 quattro, A3 3.2 quattro, my current car - A3 2.0 TFSI but to name a few).
i thought it was jerky and horrible with the diesel A3, but other than that it's been a marvel. only down points are that if you catch the gearbox napping (and you will do if you do multiple downshifts for example) it's no faster at changing than a regular slush box. it can be a little jerky around town and compared to some autos i've driven it's hard to modulate at first so you get a thump in the back if you push too hard or no acceleration at all if you're too gentle. you get used to being perfect with it tho.
in regards to the S3 specifically, someone mentioned that because it's got a larger turbo than my 2.0 TFSI it has more turbo lag, and sometimes the gearbox drops it out of it's sweet spot so you've not got that much power. definitely test drive one before you make a decision, but i doubt that that proviso will be that much of a concern because if you're dawdling the lack of boost won't matter that much and if you're pressing on you'll be changing yourself (i assume anyway). just take it for an extended test drive under several road conditions and see what you think.
as a final word - i'm only being so negative because i think the gearbox will sell itself to you. i just wanted to play devil's advocate so you know what to look out for and can make the most informed choice. i love my DSG and i'd never go back.