So far, I used the group finder 32 times. 31 times as a tank and 1 time as a healer. I only had 2 bad experiences. The first one was with a dpser who kept writing passive-agressive comments like "Do we have a tank?", "Does he know how to hold the aggro?", "Are we sure we really have a tank?", etc. I warned that dpser that I might have aggro issues since he had way better gear than me but he didn't care at all. I even had the feeling that he loved pulling the aggro. The second one was with 2 dpsers who quitted the group right after a 1st boss wipe (I think they were friends). I mean, come on! After only 1 wipe, really?!
But still, a personal 93,75% good experience is excellent imo.
The only thing I really fear with the group finder is that eventually, players will have so many people in their ignore list or will stop using it because of cumulative bad experiences, that we'll end up with fewer and fewer people willing to use it with non-guildmates. That must not be a good thing for a server community.