I'm taking "play" to mean "GM", as I rarely play and I'm okay with that...and, again, I'm giving two answers instead of one, but there's a good reason for that.
Like the TV shows, the RPGs can be taken separately or complimentary with each other (in fact, my actual favorite combo is Angel combined with The Magic Box supplement for Buffy). These RPGs hold a very special place in my heart, in part because I love the shows to death, and in part because the larger Buffyverse line holds my first RPG writing credits (including a Buffyverse related article for Eden Studios Presents vol. 2 and an additional writing credit on Angel). Three times I have attempted campaigns (one a Buffy playtest campaign, one an IRC based online game and one an Angel game), and each time they just never "clicked", even though I was super enthused and everyone said they were having a great time. I even prepared a fourth campaign, which could be best described as The Dead Zone meets Friday the 13th The Series meets Beauty & The Beast, but we never even made it to our first session. I had grown quite adept at getting the Cinematic Unisystem to do the things I wanted it to do.
I still have all the Buffy books, all the Angel playtest files, and occasionally poke around them and get ideas, but I know better than to go down that trail again *sigh*.
Actually, The Angel RPG led directly to my Savage Worlds obsession, in a funny story. I caught wind that the Angel GM screen was on clearance at a game shop and when I went in to pick it up, I saw Savage Worlds Explorer's Edition. I had traded away my original Savage Worlds rulebook (I just HATED it. The rules were bad, the characters were all the same and there was NO flavor. Yeah, I was every guy I've argued with since), but I had seen all this buzz on the internet about the Savage Worlds settings and I figured $20 for the screen I wanted (but would never use) plus the Savage Worlds rules seemed like a good enough deal...and grew hooked on Savage Worlds.
Probably one of the reasons I can't get a Buffy/Angel game going *now*.