Played 4 and a half hours today, haven't gotten Trevor yet but I'm enjoying it a lot so far. The "Hang Out" feature, like with people you know in the game, is pretty cool. It sort of develops the character you're with a little more and you get some interesting dialogue, too (at least with Lamar).
Tennis is alright, it's kind of full of dead air. In the sense that there's like zero sound effects, no real banter between the two characters going at it. The gameplay itself is fine, it's just the lack of presentation that sort of turns me off from it a little (maybe this is because I'm just used to Mario Tennis though). Not sure if that changes the more you play it (I only played one set) but that's what I think of it so far.
The shooting range is neat, not anything too amazing but it seemed like a decent distraction if you need something else to do.
Umm... not much else I want to chime in on, the story is still somewhat progressing for me. I haven't seen anything jaw dropping amazing yet in terms of storyline but I'm sure that'll change once Trevor enters the picture (as I've read a few posts ago!)