Lots of good points here made by Jesse. Given the wide variety of experiences I’ve had just doing online MTM shirts, I don’t think I’m too eager to do something like suits anytime soon. Frankly, when you factor in the time, the hassle, the waiting after paying a tailor for alterations and just the possibility of inconsistently-fitting clothing based on your probably-not-so-great measurements, it’s not worth it.
I’d rather pay a semi-premium for a piece of clothing that fits relatively well (or if it doesn’t, then has free shipping/returns or a small restock fee) than deal with the problems of online MTM.
The one exception I think I’d recommend is a service similar to MyTailor.com, where you can get measured by the tailor himself at an in-person fitting and he knows how his measurements will be interpreted at the point of production. At least you know you’re getting the measurement side down pat. I can only speak for his shirts though, not his suiting.
For the most part, once you have a good fitting shirt, you can easily take measurements off of it and test out other online MTM shirting companies, which is what I’ve done with moderate success.
There’s nothing wrong for most people to buy off the rack and then get alterations. Not everyone “needs” a custom, MTM suit.