Honestly, it totally depends on the situation as to whether or not it is worth it. I did a DNA test through 23andMe and I found the health aspect really interesting.
My mom was adopted by her biological aunt. So I know all about her maternal side, but next to nothing on her paternal side. I have a name, an occupation, and I know the man was married and possibly had two teenage sons - but I'm not even sure how much of that is correct.
I got a match that was 3rd or 4th cousins (so pretty close) and we started emailing back and forth. He has ancestors from the area in question, but none of them match the name or occupation of the supposed father. So we match genetically - but we have no clue how where the connection is exactly.
Other than that, I haven't learned anything new. The experience was interesting, but not necessarily worth the money.
However - my dad needs to be tested because we aren't sure who the father is of my great-grandfather. I've checked the court records and bastardy records and come up with nothing. His other siblings all have records in the court of who the father was, but nothing on him. My dad is the last (known) male-through-male descendant that I can test. I figure I'll enter him in the two or three suspected father's surname projects and see if he fits with one of them.
Hopefully he will someday accept taking the test so that I can truly learn who the father is.
So... in short... if you have a mystery to solve and you've run out of a paper trail - go for it! If not, then it might not be worth the money right now.