The shuttle passes are okay, but there are downsides, or at least there were. The last time we stayed off-site was back in 2006, and that was the last time we used the passes. Things may have changed since we used them, but this is my experience with them.
The particular hotel we were staying at offered free transportation to US, IOA and SW, but charged you to go to the World. So we went with the Virgin shuttle vouchers they offered at the welcome meeting.
When we did it, you paid $7 per voucher, and would need a voucher for each trip you took. Each voucher has an individual code on it that you read off to the phone operator when you book your taxis the night before. We had 5 trips to the World planned, so that meant we had to by three lots of 10 passes (5 to get there, 5 to get back) because there was three of us. So a total of 30 passes in total, which came to $210.
There are two issues I have with the shuttle vouchers: 1) times and 2) you are not alone.
1) TIMES
There are scheduled times to take you to the World and pick you up from the World. They are not negotiable, and do not fit well with the parks. There are like 4 or 5 pick-up times both in the mornings and in the evenings. You select one and off you go, but as far as I can remember there were never any good times to get you there for park openings. They may have changed this now.
Anyway, once at the park, you just call later in the day to arrange a pick-up time. Sounds great until they give you the pick-up times. For our 2006 trip, there were like 4-5 pick-up times, with the last two coming just before the park closing and about an hour after everything has closed. The thing is with these is you miss all the evening entertainment like Wishes and Illuminations. Or you can see it, but have to spend an hour sitting in a dark car park by yourselves waiting for your transport to arrive. I can remember watching Illuminations fireworks from the car park, and half watching Wishes as we ran down Main Street U.S.A backwards trying to get to monorail. We managed to catch a little bit of the end as the monorail took us back to the TTC.
2. YOU ARE NOT ALONE
This really annoyed me on occasion actually. Just because you book your vouchers, does not mean you get a taxi all to yourself, which you would if you booked it direct. And not only that, but people in your taxi were not always going to the same place as you. We wanted to go to Animal Kingdom one morning, and booked a taxi that would get us there about 15mins after park opening that morning. However, our taxi was late to start with because people at the previous hotel were late, then we and two other families got in at our hotel. We then went on a nice little trek right across the World, stopping at MGM and TL before going to AK. I think we ended up arriving 45-60mins after park opening - was not impressed.
The same can be true of the return journey too - people on your shuttle are not going to just your hotel, although to be fair, it's generally only one, maybe two other hotels they will drop off at.
If the system worked a bit more like a regular taxi service - it picks only you up and takes you straight to your destination, then maybe it would work better. It also doesn't work too well if there are lots of people in your party. If there are 2-4 then it generally works out okay. If there are more, maybe not.
As a guide, Mears will do a normal taxi cab fare from SW to Disney for around $28 (according to the fare checker on their website). That's the equivalent of four vouchers from Virgin, but you'll go direct, at whatever time you want. You will not be limited. You can see how with 5 or more, the option of a regular taxi might be better. However, if there are only two of you, then $14 for two vouchers is going to be much less that forking out $28 for your own taxi! It swings in roundabouts I guess.
Since we were central on ID last time, we just got the I-Ride to SW, and when we wanted to go to US/IOA, we got it up to WnW, then walked across the bridge. It's only like a 10-15mins walk. On a couple of occasions, we did get the I-Ride to SW, then the Lynx bus over to Disney, and used the Virgin Shuttle vouchers to come back. There are plenty of options.