I just flew this past week from Detroit to Tampa, and back, on Spirit with a backpack. I can say that the flight out from Detroit was completely full, but that nobody was enforcing the personal item size requirement at the gate.
Leaving Tampa was altogether different; the flight was nearly full --about 3 or 4 empty seats, I reckon-- but there was a firm yet polite gate agent that stated in no uncertain terms that the sizing policy was to be followed. There were a couple of people that did not board during the zone 1 phase, and the agent asked these people who had obviously oversized personal items to come measure them in the sizers. He asked them, after the items didn't fit, of course, if they were aware of the carry-on pay policy. I had to keep a straight face as one man feigned obtuseness to the policy, then was clearly upset about the impending $45 he was about to spend.
What, is he the only Spirit flyer to not know about this?
I should say that this flight was delayed by about 20 minutes coming into Tampa, so the agent was also giving instructions to try to expedite the boarding process, i.e., zone 1 was already lined up when the passengers getting off the inbound flight were deplaning. This agent did a terrific job, by the way, and the plane arrived in Detroit about 10 minutes early.
Maybe the delayed flight caused the policy to be enforced? I don't know.
As mentioned before by iahphx, it seems like this policy will be a YMMV kind of thing, although once people catch on to this, they may try their luck at attempting to *sneak* on bigger things.
FWIW, my backpack was slightly under the size requirements, and I was able to bring enough clothes for 3 full days, although I had no pants, sweaters, or "bulky" items. Had strictly t-shirts, shorts, and socks to deal with the heat!