I'm running a 17x9 +42 Rota on my ms3. I don't think it looks *terrible* but the car does need a drop. Keep in mind that the stock 3 hatch/sedan were only 17" wheels, and they look very respectable, at least IMO.
Now for the fun stuff- Why did I do it? I made the decision after deciding what I wanted to do, and that was stuff as much tire as possible under the car. I was willing to sacrifice a little bit of look for the sum total of a wider tire. I also happened to lose some weight on the deal (only 18lbs/rim) so that was a bonus. The wheels were also cheaper than purchasing a set of 18" wheels, at least new, and the tires are going to be cheaper. I also will have to worry less about rolling my fenders, at least until the Konis and Eibachs arrive. All in all a good idea.
Now the bad part- You have less wheel choice. 17" wheels seem to be restricted to mostly 7.5" widths, which was not enough to satisfy my needs. I needed at LEAST an 8" wheel..... because I was fully intent on a 245, and possibly a 255. This restricted me a GREAT deal when it came to style/color/offset. Most of the wheels I saw that were an 8" were like a +50 offset, and that REALLY kind of bothered me. I didn't want to go that high due to concerns of rubbing on the inside. I can roll and pull fenders, I cannot move my suspension. Took me a solid 6 months of searching for wheels, and at LEAST two dozen sets of potentials before I bought the Rotas. I juggled numbers until I was nearly banned from a local club (but I'm a mod there, so really, they couldn't have done much to me) and the solution was- go wider. This allowed me to get into a lower offset, and push the rim further out. Sure, I'm closer to the suspension than I'd really like, but not as close as if I were to run say a 245 on an 8" at a +50. All in all, it was a giant pain in the ass.
That being said, I say do whatever makes you happy. If you're concerned with performance, go with a 17", the weight savings and cost effectiveness are a blessing. If you're more on the looks, an 18" is going to be your horse. DON'T go 19", because tires are retarded expensive, and limited as hell. If you go bigger, well, good luck to you.
