Kiyoshi brings up some great points that I neglected to mention. The MTM process is not as instant and perfect as these companies would like you to believe. For this reason, I generally only consider companies that offer free remakes of your first shirt, thus allowing for some wiggle room as you begin honing in on your measurements. Ratio and Proper Cloth both offer this service (I’m not sure about Cottonwork).
Ian, aka fromsqualortoballer, wrote up his experience with three online MTM shirtmakers. Worth reading if you’re thinking about using these services.
My personal recommendation is MyTailor.com, because their tailor, Joe Hemrajani, travels to measure you in person. Otherwise, I recommend finding a local shirtmaker where you go do multiple fittings until you’ve nailed it down (if you’re in the San Francisco Bay Area, I recommend Spoon Tailor).
I also highly encourage you to stick with a MTM shirtmaker after you’ve honed in on the fit — it will likely take several shirts to get the fit perfect if you’re just measuring yourself or off a well-fitting shirt. I’ve made the mistake of hopping around and ended up just having to start all over at step zero, which is a frustrating waste of money.
It’s worth considering that your first, well-fitting shirt will cost you $200 (+/-$50 to $75) just because it’ll take that many trials to get the fit correct. Also, just because you enter the same measurements from one shirt into another form, that doesn’t mean each shirtmaker will interpret it the same way.
Of course, all of this isn’t even taking into consideration that you don’t know the quality of the cloth being used or the quality of construction until you receive (and pay) for the final product.
For all of these reasons, I really am hesitant to push people into online MTM shirting. Once you get it right, it’s great. But the process of getting there is highly time consuming, frustrating and pricey. While a lot of these companies try to make it easy as possible, it’s far from flawless and no panacea.