We have the Chariot Cougar and we love it. There are different models with different features (and prices). You purchase the main chassis (seat, cover, 2 wheels) and then get the conversion kits you want - they have strolling, jogging, biking, hiking and skiing. We have the jogging and biking kits and they both work well and are REALLY easy to switch. The cockpit part is really nice with a good amount of room and a generous height allowance. It seems to ride really smoothly and has a good suspension system.
I researched these quite a bit before buying and the Chariot seemed to be the best choice for us - great quality (the Burley by comparison seems very cheaply made, especially the cockpit part), versatile and it got great parent reviews. It is pricey, but when considering the price of a double jogger and separate bike trailer, it is totally worth it. We compared to the BOB Revolution dbl jogger and it is actually lighter and the dimensions are smaller, even though the seating room is larger. I was really surprised by this.
We purchased ours at REI during their recent sale (20% off all Chariot products) and they had them in the store to check out. However, I recently moved from Chicago to Utah so I'm not sure about the local REI store stock. You can call to find out if they have them to test.
Also, for what it is worth, the first time we took it out on the bikes to a popular local trail we saw sooo many other Chariots. There were a few other trailer brands but the Chariots by far outnumbered them. This is a very active area and it seemed to be the most popular choice with other parents here. The REI reps said they sell a ton of Chariots, as well.
My 18 month old loves sitting in it and going for rides (we have another on the way).