The first one's well worth your time. Essentially, it's one part science fiction, one part horror, two parts 80's action flick. Long, slow suspense broken (at ever-shorter intervals) by moments of intense (and sometimes over-the-top) violence.
It was shot in the jungles of Mexico, so the scenery's not bad either.
The second one isn't as good -- mainly because the scriptwriters decided to write it using the premise that urban society was going to collapse into near-barbarism within a few years, which makes for some hilariously over-the-top moments.
However, the Predator has more high-tech equipment than in the first film (added: net, spear, boomerang-style disc, small-scale speargun), and an early action sequence where the Pred puts all its hunting gear through a "test run" is loads of fun...in an ultraviolent sort of way. There's also some more characterization for Preds and Pred customs/culture -- for example, the brief "trophy wall" scene late in the film was a WONDERFUL moment.
You should definitely watch the first one, but the enjoyment you get out of the second one is going to depend on how much you liked the first.
Dialogue in both films is alright, but not spectacular. Good sense of humor, though -- the one-liners are actually funny most of the time.
Probably don't have to say it, but you can skip the pair of recent "Aliens vs. Predator" films. They're both crap -- I skimmed the first one, and didn't even bother with the second one.
Actually, I think I'll go watch that "test run" scene for a minute...