I've used them a bit. They're OK, but not that great.
Pros:
1. somewhat cheaper initially
2. take up less space when not in use
3. May be able to transplant with less root disturbance by sacrificing and cutting open bag.
Cons:
1. Flimsy compared to standard nursery pots. Probably will not last nearly as many seasons. May end up being more expensive in the long run as a result.
2. A bit more difficult to work with in terms of filling with mix, since they don't hold their shape too well (once they are filled with potting mix they hold their shape fine).
I probably won't buy them again unless I really find that storing the standard gallon pots becomes an issue, or if I desire the ability to cut them open and sacrifice them to minimize root disturbance (not likely).
Rob