I don't have any real stories to tell, as I only experienced three different GMs in RL. One was a classmate, and I guess he wasn't were very good, as I don't remember much about the game sessions apart from the players kidding around all the time and the characters not really getting anywhere. When we'd travel from one point to another, time would pass in RL, and when I asked: "So what's happening now?", he'd just reply: "You're still travelling". It didn't feel like the adventures were very structured and the GM didn't really take control of the situation. I guess he'd tell the players to stop kidding around from time to time but to no avail. Then again, we were all just teens with no prior experience of roleplaying.
I also tried to play with people I didn't know once, answering to an ad. I went there two times, first to create my character with the GM, then for the first session, and then I got told the campaign was cancelled. I never really learned why but it sounded somewhat strange to me and I suspected that the GM just didn't like some of the players and wanted to continue without them. I don't think I gave him any reason to dislike me, but who knows. To tell the truth, I didn't feel all that comfortable playing with the others either. I never tried to apply for a game with total strangers again.
The third GM was actually a great one. I already knew him and the group before we started to turn our meetings into pen and paper session, and it was a lot of fun. The custom settings and adventures were well-planned, the GM was flexible and able to react to our characters and ideas, it was never boring. The only thing I didn't like was a situation during the second campaign, which I already mentioned in that Favorite Spell thread, when he had planned to have the whole group imprisoned, executed and resurrected by a mysterious ally. It was all part of the story he had worked out, but of course we players didn't know that and naturally fought for our lives. The others got captured but my character gave the pursuers the slip and subsequently started several attempts to free the others. The GM played along for some time and it was great, but for some reason or other all my attempts failed. Eventually, he took me aside, revealed what he planned and appealed to me to just let it drop and get captured, too, so he could move on with the story. So I did him the favor, but I was quite disappointed, as it didn't feel right that the execution of our characters was part of a fixed story, I mean it's just a game and in the D&D universe resurrection isn't such a big deal, but to me it was - it didn't want my character to die and get resurrected by a mysterious stranger if I could avoid it, and I did avoid it and could have saved the others from that fate too, if it wasn't for the GM's appeal to play along. Even though, in general he was a fantastic GM, I still feel a GM shouldn't purposefully force the players to die in this way, just for the story. That's kind of a sacrilege.