On the whole they're not really worth it.
You get a lot of really crappy sports/puzzle/RPG games that you will never play and are hard to sell.
Depending on the seller, a lot of doubles. It's not unlikely to receive 10-30 doubles.
In the case of CD based/boxed games, you often receive a lot of damaged cases, or empty cases without cart/discs or manuals etc.
The games are filthy! It takes a long time to clean 100+ games.
A friend of mine recently picked up a bulk Famicom lot of 100 games. In it there were 15 doubles, 55 worthless crappy titles and 30 average inexpensive games that he added to his collection. He ended up selling some of his doubles for $2 a pop and recouped most of his initial outlay, but it was a big hassle to end up with 30 generic, common games.