I did Habitat one summer for the company I was interning for (a group of us went as a team) It was MAD hot and they had us outside painting siding and doors, moving toilets, etc. Then it rained an hour before we wrapped up. I came home sweaty, wet & covered in paint, but I had so much fun and met a lot of cool people, I wanted to do it again. I was supposed to a few weeks back but they cancelled my build date and my schedule didn't work out for the next two dates
It was definitely worthwhile, I felt real good (probably didn't smell that way, though) after it was over.