I did not even know they charged to "choose my seat" until after I committed to buy tickets with them. Between that fee and the luggage fees, I would likely never use them again.
I have flown Spirit several times, and liked it fine, the planes and crews were great.
I don't think they hide anything. Charging for luggage is so common now, the few airlines that don't do it make sure we know they don't

.
Just for fun, I did some comparison-shopping. It's no secret I love Jet Blue, so I picked a random set of dates: May 1 to May 9 (because I didn't like their return fare on the 8th

) and chose the same for Spirit, Boston to Orlando, one person.
JetBlue's base fare down is $129, back is $119. With all taxes, round-trip is $280.10. That's it. Unless I want to upgrade to an Extra Legroom seat for $20, there are no extra costs. I can check one bag and carry one one small bag and a personal item. I know all this, but it's also explained to me on the JetBlue website - the seat cost when I select seat 3D, and the other information is just 'there'.
Spirit's base fares are $79 each way. With tax, the total base fare is $209.80. One carry-on and one checked bag in each direction will cost me $28 if I join the $9 fare club, $58 if I don't. Now my fare is $265.80/$295.80. Of course, it will cost me $59.95 to join the $9 fare club

.
Okay, now, the next page - granted, I've provided traveler information but I haven't committed to anything, I haven't given any payment information - is about seat selection. I can pay another $75 for a Big Front Seat, or between $10 and $25 to select my regular coach seat now. It's pretty explicit. The text states "Pre-select your seating in order to ensure that you get the seat that is best for you. Want a window seat? No problem!
Sit next to your traveling partner? You bet! More legroom? Sure thing! - You are an individual. Go ahead, treat yourself as one." and the seats are color-coded.
Now, granted, in small print at the bottom of the page there's a statement that if you don't want to pay to select your seats, a random seat will be assigned to you at no charge when you check in. But the information is absolutely there, and it's clearly offered before any passenger commits to anything.