I've had a lot of positive experience with various military surplus ponchos. My ex-German army poncho (green rubberized type) has accompanied me on a good many outdoor treks over the past 15 years, still going strong. It's absolutely waterproof and can be used to make a very decent lean-to (or equivalent) shelter.

Wearing a poncho in a warm climate gets hot quick, so you will sweat a fair bit after a while. Especially on a hard trek with a lot of physical exertion. Though it's no worse in that department than a rain jacket or anorak in my experience - those things tend to be even hotter because there's less ventilation.

As noted above, a poncho will not protect your legs very effectively, particularly if you're a tall guy like me. One solution is to wear waterproof pants or leggings, but I prefer gaiters instead. Works well for me.

I have not really used commercial ponchos much myself, but I've noticed the ones used by some of my friends (I think one of them actually has that Snugpack poncho) aren't quite as waterproof as the inexpensive military surplus stuff.