I like GMPCs, personally. If they're support characters who never make any important decisions and don't railroad the PCs, they can be great. Heck, there's been a GMPC in nearly every D&D campaign I've played in.
A lot depends on the players. For instance, the last campiagn I ran, there was no dedicated warrior-type in the group, so I brought in Sir Justin, sword-and-board fighter. He was the flat-out best and straight-up fighting, but the players - squishy casters and rogues, and one cleric - appreciated this immensely. My players tend to be very tactically-minded, and would rather see the GMPC get his "moment of cool" than do without the extra resources. It helps that the monsters are utterly impartial...they'll kill whoever's closest and tastiest.
One oddity [?] about my group is that we often "guest GM." A player will have an idea for a one-session adventure, so he'll GM in the normal GM's campiagn for a session...while the normal GM plays the GMPC. Kind of like the Sanctuary series in gaming form. For this reason, the GMPC is even more useful.