I caught Slayer live twice. Late 90's/early 00's. The first one was maybe a 4,000 person venue (ice hall), and it was cool though I didn't know their material that well.
The second time was in a festival and I was already a semi-big fan (seems like I never took the final step into total fandom). That was the God Hates Us All tour. I got the whole experience, moshpit and tour t-shirt and all.
I always liked a couple other thrash bands a little better (eg. Testament, Annihilator), but I do own 4 of their CD's and still think of them fondly. I think they've contributed a lot, and eventhough "farewell tour" means nothing nowadays, I think that means I won't be having much to do with them anymore, as I haven't bought a CD of theirs in ages anyway. I guess they keep recording, maybe.