It all depends on your expectations. We flew spirit last summer, and although I think the $1,000 in savings is well worth it, my husband will never fly Spirit again.
They do charge you for every little thing (checking bags, booking seats online, onboard snacks. etc.) Also, it is very hard to find someone who speaks good English, sometimes it can be frustrating when calling about your flight (plus there's always a long wait.)
The seats are very close to each other, and we found our plane to be filthy.
HOWEVER, if you go in expecting it to not be wonderful...I think that the money saved is well worth it.