I think it is a flop considering how long it took Micrsoft to develop, and you would have thought it would have lasted 5-6 years before they released another OS.
ERrm, MS make no money supporting old software, or relatively little, they make money, releasing new software. All software people do, the goal of any company, especially a one with basically a monopoly on the market, is to release as many damn software releases as possible as each new one generates.......... money.
A failure is an OS that lasts too long in that sense. XP was a failure in terms of profitability, it was too good, lasted too long and the big changes for vista took too long to make.
Vista was fine frankly, it might be a little to enthusiastic shall we say, in terms of prefetching data for cache, but its a good solid OS, and for me even though XP was great and very stable compared to old OS's, Vista was still the next level up of stability and thats even with a lot of duff drivers about for a long time.
Its biggest failure is people complaining about memory usage, which mostly was just cache issues and is similar to most other OS's. Vista "appeared" to use more memory than XP, which is does a little but most of the overhead was cache rather than wasted and is the same way OSX and other os's handle memory. XP simply wastes stuff there to be used and it uses power either way. The higher memory usage is mostly what people would use as an excuse for why it sucks and needs a better system, etc, etc, etc.
In reality I've seen systems with Vista crawl, and I've seen systems with XP crawl, its more about user and stupid crap installed than either OS tbh. Both will run on very very old systems just fine. Both running smoother and quicker with certain things removed/disabled, Windows 7 will run smoother with things disabled, thats life.
The one thing MS really need to do is ditch the 32bit OS's, no one needs it anymore, and lack of it will push people to do better with 64bit drivers and half the work needed to support a new OS, which will make dev's more likely to support it quicker. I don't think a chip in what, 5 years has been made that can't run 64bit so at this point they just need to kill 32bit, it will make everyones lives so much easier.