Here's an excerpt from my recent post on StyleForum.
In my book, if you do not fit RTW or even MTM very well, then the end product of bespoke is worth every penny. But of course you need to reach that level of realization for yourself.
Some ways to increase the likelihood of success when you go bespoke:
Do your due diligence (i.e. know what you want and match that as closely as possible to the tailor most likely to capture your needs and preferences). If you do this, I contend it is entirely possible to be anywhere from highly satisfied up to insanely happy with the first suit from a bespoke tailor.
Build up your knowledge of tailoring, garment features and styles over time. I went through RTW, MTM and bespoke in that order. I do not suggest diving directly into bespoke without a decent sense of your stylistic preferences and adequate knowledge of tailored garments.
Learn on the cheap first (ebay, filene's, sales, SF, AAC, etc) then pull the trigger on bespoke.
Basically, the more you are familiar with what you want and need (and which tailor can optimally supply it), the better value and satisfaction you will receive both aesthetically and practically speaking.