We have has success flying with Spirit and don't mind them. Its $59.95 a year for the $9 fare club. It gives you discount on flights, bag and in flight snacks (i think). I usually don't enroll in it until I find a flight I want, I will cancel it at the end of each year that way they don't auto enroll me again. This year our flight for three was $413.34 + $59.95 + $80 (2 checked bags) = $553.29 total. I know in previous years I got flights to MCO for less, I think around $450 with bags.
You can sign up to receive notification of their cheap flights on their home page at the top. They will send you emails just about every day for different times of the year to different places. If you see something you like though you usually have to act fast.
They will charge money you if you want to pick your seat before you leave. You don't have to do this, I only did it one time. All our other times I didn't and we've always been seat together no problems. Even with our party of 12 that went to NY by Spirit was seat pretty close to each other, we had smaller groups but I think we were all within 5 or 6 rows.
As for the fees for bags, its cheaper to check bags then to do a carry on, $20 versus $25, I prefer to checked bags. I personally don't want to have to worry about watching extra items. You have to pay for your bags before you do your online check-in 24 hrs before your flight or is more, other wise is $25 checked and $30 carry on, when you pay for them at check-in. Crazy I know but if you can follow their rules, they have cheap flight deals. Checked bags can only weight 40 lbs too. Each person is allow a personal item (backpack, purse, tote or laptop) that has to fit under the seat in front of you.
I think that's about it. I've never done an international flight with them. I do know that if you wait to do anything at the check-in counter or your bag is more then the 40 lbs, they have crazy fees.
I suggest just looking around their website and signing up for the $9 flight notifications, its free and can't hurt.