I have one. I've been using it for almost four years, now. I'll write you a detailed response because I don't have much to do right now.
Transportation pros:
1. I use to have a bobelock bag and carried around my bass with the shoulder strap. It messed up my back and I knew that I needed to change, so I changed to the double trolley. It cured my back problems, and I now have no pain in my back what so ever.
2. I can attach my tuxedo or what have you to the case, freeing up an arm that can open doors, press elevator buttons, etc,. I generally hate walking to a gig in my tuxedo, but I'm getting tired of attaching my tux case to the bass case as it can be annoying.
3. I can walk very, very fast with the bass. If the wheels are filled up with the proper amount of air, the double trolley can really move. This is great because I walk fast and I always get to a gig or rehearsal an hour to thirty minutes early.
Transportation cons:
1. The biggest problem with it that I've found is in the wheels. I've replaced the tires on the double trolley twice now, and the flange on the wheels is too big for the tire of that size. I brought it to the bike shop both times (same bike shop) and they had a lot of trouble taking the tires off and putting new ones on. So much that they were scared they'd break the rim (even though they didn't).
2. There isn't much to hang on to while wheeling the bass around. When I received mine the straps attaching the bass to the trolley were not there (Marc gave me an extra year of warranty and when I accidentally filled a tire up too much resulting in a broken wheel, he sent me a new one, free of charge) so I bought some bungee cords. They work well, but they tend to put tension on the bass and you often end up wheeling the bass with your hand on the heel of the bass, under the bungee cords so you have something to hold onto. This can hurt after a while. I've walked probably 5+ km to a gig with the bass and while it isn't too bad, it can be really annoying.
3. The thing is heavy. If you have a heavy flatback bass or a big 5 banger, you will probably have a hard time. I have a big 7/8 bass and while it is manageable, it wasn't easy at first. Ironically, my back, core, arms, and legs are now stronger from bringing it up and down stairs. You learn how to maneuver it so that you can do this fast, but it takes time.
4. It is kind of large. I live in Montreal, so I almost always have to take the subway everywhere when I'm with the bass. It can fit onto a bus with the trolley, but you better hope there is space where those seats recline into the wall or you're gonna have a bad time. I almost always walk instead of taking the bus if I feel the need.
5. The trolley requires maybe once or twice a year maintenance of tightening screws / checking the parts.
Stool pros:
1. I have my stool wherever I go, and I never have to get use to another stool's height that changes the set up / angles for me.
2. The stool is highly adjustable so I can change how I sit when I feel the need. I almost always change how I sit every 4-6 months because I find sitting gives me back problems and my bass is big so I'm constantly trying out different ways of sitting.
Stool cons:
1. Sometimes the stool isn't very comfortable. This is because of how the stool is cut; instead of a large circle like most stools, it is almost a triangle. That means my ass takes up the whole seat (I'm a small guy, too) and depending on how I'm sitting, I'm not 100% on it. I have deduced that it largely depends on the underwear I'm wearing, the pants, how hairy my legs are, and how high the seat is. I find that the higher the seat is, the less comfy it is. I think it's because I can't put both feet on the ground to take some weight or spread it out. Sometimes this makes me need to stand up for a little bit during a performance because my ass is sore. This is the biggest con of the whole set up, and it went away when I sat as low as possible with my bass, but sitting that low with a large bass caused some technical issues with the bow and jazz pizzicato.
2. There isn't a real footrest. If you're wearing shoes or boots with a heel, it is fine because there is a place you can hook in with your heel. I like playing in my fancy boots because of this, but putting your barefoot (with just a sock say) can become uncomfortable.
3. Since the seat isn't big enough, it can make the back of my highs (the higher part, almost where your legs meet your butt) pretty sore after a while.
I think in the end, the double trolley has many flaws, but the greatest pros almost always make up for them. I've gotten to the point now where I've almost shaved the back of my legs to see if that is the root of the discomfort, and I feel like I'm almost at the breaking point of giving up the double trolley because of that. If I had a smaller bass and could sit lower without the bass nearly horizontal, it would solve that issue (less weight on my ass and generally falling on the upper back part of my thighs. I experienced this while playing an opera recently (after five hours of playing) and I stood for a few movements. When I sat back down all the soreness was gone, but I think it was because I was sitting slightly differently.
What I really think is that the single wheel is the worst thing you can get in terms of transportation. You can't attach your tux, and good luck if you want to bring a stool with you. With the double trolley I can put my tux on the bass, put my amp in a backpack, and wheel my bass around. That is perfect because I have my stool with me, too! And no need for a car. However, if I had the ability to play standing well (I have tried many times and initially started playing standing) I would instead trade the double trolley for back pack straps.
This might seem like a negative review for the double trolley, but it has its shining moments.
1. I always have my stool.
2. Transportation is easier and much, much safer than the single wheel.
3. I can attach my tux and carry my amp on my back.
4. I never have to sit on a REALLY uncomfortable stool that is A. Much too high B. Much too low C. cuts off the circulation to my legs or D. the conductor takes the stool instead and I have to play the whole time standing, hoping to god that I can play Beethoven 3 like that.
I'll end with this: if I was hit with the question of buying the double trolley again, I would answer like this:
If I was a sitting bass player and felt like there was no going back to standing, I would buy it again, no problems.
If I was a standing bass player who sat sometimes during long rehearsals for comfort, I would not buy it again, and would instead invest in an awesome case with back pack straps.