I think that Rattay's a wise pickup also, but I think it's too early to judge Rattay vs. Simms.
There are reasons to think that Simms will flop, among them his getting injured fairly promptly during his first-ever NFL start and his lackluster second start against the Arizona Cardinals last year.
OTOH, Simms has performed well during the preseason and during relief efforts, as with the job he did against the Dolphins. The key issue seems to be whether or not he can keep his head on straight during a game. It seems that the anticipation of starting gets him so hyped up that his performance suffers as a result. This was an early problem for Brett Favre also, IIRC. This is not to say that Simms is another Favre, of course. Favre has had remarkable durability over his career--durability that Simms has already failed to match even if he starts every game over a ten year period from here on out.
I think that after five games with Simms at the helm we'll have a good idea about how his career will turn out. If he turns in a solid regular season, then we may even get an indication of whether or not the rap against him for wilting under big-game pressure will follow him into the pros for his whole career.
A qb can be considered a decent NFL qb and also a big-game liability.