They have different priced plans for the movie pass, $9.99 trial is for one at a time, per month. The second month on bumps to $14.99.
They have a 2 at a time and a 3 at a time pass at incremental charges, $19.99/mo for 2 at a time I believe. They don't have a limitation on rentals, in theory you could rent a movie, watch it, on your portable DVD right there (or outside in your car), go back in the store rent another, repeat all day long. Same with the 2 or 3 at a time options, you can go back multiple times/day if you had time to watch them all (or just decided not to watch one).
There were no restrictions on titles of movies to rent when I used it a few months back. This was before the no late fees change/promotion so not sure how that impacts things.
The only caveat they had, and they were clear about it, was it auto-renews unless you cancel. So you have to watch your receipts for the last day of the plan and cancel a day or two before hand (they let me use the last day even after the plan was cancelled).
I'd probably do it again, at $4+tax per movie, it pays for itself very quickly. Only cost is gas/time and if you don't live that far it's a great deal.