I've been a member of Dollar Express for three years now, and have not had one complaint. I have rented several times at MCO, and while there's never been that long of a line, it is nice to have the ability to go right to the Dollar Express lane and be able to have the next available counter person take you.
The only complaint I would have is that every time that I've rented at MCO, I've never received a van that was parked in the Dollar Express spots, which are closest to the in-garage rental counter where you get your keys. I've never had a long walk, but why wouldn't they simply give you a vehicle that's been parked in the Dollar Express spots.
On a big positive. The minivans that I've always rented from Dollar have had 4K miles on them or less, and have been very clean.
It's Free, so go ahead and sign up. You're not losing anything by doing so.