No, Alex is correct. It was neither buggy nor unstable by the time Service Pack 2 released, quite contrary, and it was just as snappy as 7 ever was. On adequate hardware and equipped with a SSD, it was a very enjoyable OS, and I dare say better than whatever garbage 10 or 11 Microsoft has us deal with today. I believe that it's probably where Windows peaked as an OS entirely, and if you ask my personal opinion, Windows Vista Ultimate was probably the most complete edition of Windows that Microsoft has ever shipped. Windows 7 is nothing but a reskinned and slightly stripped version of Vista, the highlight essentially being the WDDM 1.1 improvements and the fact that UAC isn't a "full security" and "no security" toggle anymore, with the middle-ground option being then chosen as the default ever since.
The problem had always lied with OEMs releasing computers that were simply not powerful enough to provide a decent experience. I recall my mom had a 2 GHz, single-core AMD Turion64 laptop that was sold as "Vista Capable" that shipped with Home Basic and 512 MB of RAM. If I recall correctly it had a Radeon Xpress chipset or something like that, and it was barely fast enough to run Aero. That's why Vista "sucks". These were the computers the largest majority of people had at the time, and these were good for Windows XP at best.
By early 2007, the fastest computers you could buy were equipped with the Core 2 Quad Q6600 and Extreme QX9700, a little later with the original AMD Phenom processor. Needless to say those processors that we fondly look as relics of the past today were insanely high end and insanely expensive beyond the means of the vast majority of people at the time.