The pregens from the 4th edition Anniversary book are mathematically sound, but they aren't optimized to the degree that even novice players to the system tend to make characters. As a rule, they're fine to use for 1-shots as long as 1) the group is new, and 2) they ONLY use the pre-gens. Combining pre-gens with custom characters is asking for trouble. The pre-gens are generally good enough against most street-level NPCs...if you're going up against gangers and such (see also: Food Fight), they'll be fine. If you're going up against Red Samurai, you're sasquatch chow.
If you want to use a more high-powered group of pre-gens, the character assistance subforum on the official SR4 boards (NOT Dumpshock!) has a large list of more...optimized...pre-gens that aren't totally broken. I prefer, though, to start brand new players to SR with the "official" book pre-gens. First, they have nice art. Second, they reinforce a major concept in Shadowrun that you shouldn't depend on your stats to carry you through a run - you should depend on good tactics and a good plan. Not having the stats there to fall back upon reinforces that concept.