Well I'm one of the few advocates for Spirit on this site, and I have never had a problem flying with them. I have never had any "hiddden" fees tacked on or anything of the sort. It is all pretty straightforward. Remember, you don't always have to pay for a carry-on with Spirit. If it fits under the seat you are not charged for it. I have never had any problems with it, nor have I been charged. A lot of times they don't even check, and you can even sneek on with it if it is bigger. I once brought 2 checked bags for free.
You don't get amenities, you pay for everything even a coke. If that's something that's important then you'll have to pay for your drink and peanuts.
The flights are fine and the seats really depend on the aircraft you get. I've flown flights with them where your legs are pretty cramped and your knees are touching the seat in front of you and I've also had flights with them with quite a bit of legroom, just as much if not more than any other legacy airline (unless you've got economy+ of course, but idk what United did with that after the demise of my poor continental).
All in all, if you know what you're getting into, it is definitely worth the money saved if you calculate it out and it is less money. Flights aren't delayed any more than they are with other airlines etc etc.
If you're a business traveler, it really isn't something I'd recommend. There's no business class and what's the point of being a frequent flyer if you can't get an upgrade.. even though it seems like you need Diamond to get one now
It is great for weekend getaways if you want to do something nice and cheap. I've flown with them quite a few times within the last year and also some time before that internationally.
Hope I cleared things up for ya