The T-90 is only called the T-90 for political and export reasons. It is, at it's core, an upgraded T-72, with protection and fire control upgrades from the T-80. The only reasons it was named T-90 are #1, "partly" due to the upgrades, and #2 "partly" due to the poor showing of the T-72 in GW1 and other earlier conflicts. The Russians were scared they wouldn't be able to sell anything named T-72 to export customers, so they renamed the tank, and played it up as a whole new tank, which it isn't. Their own Army didn't even want them, they wanted the T-80. Also, in Chechniya "Large numbers of virtually identical (to the T-80) T-72s were lost in the conflict, and there is nothing to say the T-90 would have fared any better." The T-90 was forced on the Russian army because they are cheaper than the T-80, and were only intended as a "stop-gap" until a new tank could be designed and tested. The T-90S, which is the main version in use now, has "similar offensive and defensive capability as the T-80." The only basic difference is the T-90 has a diesel engine instead of the turbine in the T-80.
* Above information and quotes from "Tanks" Chris Chant, Summertime Publishing Ltd.
In essence, the T-90 is still at least a partial generation behind the M1A1 and Leopard 2, and most likely a full generation behind the M1A2 (especially the SEP) and the newest Leopards, and most likely, the newest Merkavas...
The Russian tank that should be compared to those is the new T-95 they are working on, although it is still in the design stage the last I saw...
T-95 (Objekt 775)
Here's one fact that I find fascinating: During the Gulf War, M2 Bradleys destroyed more Iraqi armored vehicles than the M1 Abrams. This is from Wikipedia, but I have seen similar statements in other publications... This doesn't have anything to do with the T-90, I know, but I just thought it was one of those "Wait... What?" pieces of trivia...