I've had both good and bad experiences with this scenario. If you play a few songs with new people it sometimes works out but if you're going to do a three set gig it can get dicey. Go for it! But be prepared for some rough spots.
Wow! This post reads like a horoscope!
I lol'd. I was asked to sit on on drums this past new years for a show. There was actually two shows booked, one was an hour and another was about 3 hours, the 3 hour one got canceled at the last minute. Anyway, they gave me a copy of the cd and I learned as much of it as I could. The show was in like 3 or 4 days. Luckily we got a practice in before hand, which only was us playing about 4 songs.
Anyway, the players in the band told me to not really pay attention to the cd as they were a jam band and made transitions by a head nod. Now, me being into Dream Theater, Meshuggah, etc. found it, for lack of a better word, unreliable to depend on a head nod to make transitions in the arrangement, I'd much rather read from sheet music than, "switch to the chorus when I "feel it". Me and the bassist jammed before a few times in the past, so we locked up pretty well.
The day of the show I was basically a nervous wreck, listening to the cd the whole ride down. We got on stage, I haven't played a show a few years, mind you, and the sax player got lost, so we had to play without him. We played about 8 songs, of the 8, I knew about 3. That's the only ones we could play because the absence of the sax would make the other ones sound naked. So, they totally surprised me by saying on stage, "yeah, we're gonna play a few blues tunes, give us a blues beat". We played two blues tracks and a Grateful Dead tune, and I never heard any of the covers I was playing.
All in all, it turned out really well, better than I expected. Even though there were only about 30 people at the venue, hahaha.