Definitely 100% certain there is LOTS of legroom in Spirit's Big Front Seats. And certain (as you've already learned) that you must store any items in the overheads.
Here's the problem with that: Spirit has boarding zones, and Zone 1 is reserved for folks who pay for carry-ons. I recently had an experience on a flight where I boarded in Zone 2 with my small backpack (legitimately "personal item" size, not so big that I needed to pay for it as a carry-on) and of course had no choice but to put it in an overhead bin. HOWEVER, people who boarded before me had hogged all of the bin space near me. I had to out my backpack in a bin over Row 6 or 7. It wasn't such a big deal getting it during flight, the problem was I had to stow it again for landing and getting it back after landing was quite difficult. These weren't the friendly skies, I had trouble getting it since after the plane parks everyone jumps up to line up to exit the plane. I would've been the first off the plane (a nice perk of those seats) but I had to swim upstream to get my stuff.
I suppose if you also have to pay for a carry-on then this wouldn't be a problem.