We have a Tandem Island that we use in kayak mode often, and it is very nice in kayak mode. When we first got into Kayaking we purchased a Hobie Revolution (for her) and a Hobie Oasis tandem (for me). We car topped them both and traveled all over the country for years, going out wherever we could launch. When in Colorado we went down several mild rapids in the boats on excursions. We purchased the sail kits for both boats, and sailing turned out to be the most fun for us, we always carried the sails on board, if we got any wind we would raise them.
In 2010 we traded in the Oasis for a tandem island. I used the same Malone roof rack systems that were used on the Oasis. To be honest it is easier to load the TI onto the roof (by myself always) than it was to load the Oasis. The TI has handles on the side that make it easy to lift the front half(about 50 lbs) onto the T bar hitch reciever on our Yukon Denali SUV. The front half of the Oasis was around 40 lbs, the TI is a couple feet longer but it's hard to notice the length difference. The TI as a kayak is superior to anything I have used (even solo). To give Hobie credit, the Oasis was the older design, last year they came out with a new design, that is way superior to the old design, so most of the problems I discribe are no longer valid.
Funny story when we used to go out together with the me on the Oasis (solo) and the my wife on the Revo, my wife on the Revo would accelerate an pedal around me circling my boat taunting me to keep up (I was doing the best I could do). The Revo is so nimble and fast compared to the Oasis (a much larger boat designed for tandem use). Now I have the TI, running solo my TI is much faster than her Revo, and for payback I run circles around her taunting her to keep up. However I tried in on a narrow river, and we ended up both going over (the boat is after all still pretty long, and a little more difficult in close quarters, or narrow rivers). We ran the Huron river rapids in Ann Arbor, MI with no problems with the TI (Kayak mode of course), (it gets pretty narrow and windy in a few areas), no more difficult than the Oasis was.
I'm pretty certain that the Tandem Island kayak is hands down the fastest kayak in Hobies kayak fleet (solo or Tandem).
To be honest I have not paddled much, the Santa Fe river in Florida (where the springs are) has a lot of weeds during certain times of the year, and it is easier to paddle (the mirage drives and rudder are like magnets to the weeds). And of course we remove the mirage drives whenever we run rapids. We actually prefer going tandem when paddleing (so she can pretend to paddle in the back while I do all the work LOL). Tandem the TI is a dream, very balanced, and with two peddlers/paddlers it just flies effortlessly. The Oasis was a divorce kayak when paddling tandem, we would argue like cats and dogs trying to navigate and coordinate our paddling (true story). We can easily cover much greater distance with much more storage with the TI.
We live near the Ocean, and mounting the AMA's and the sail turns the boat into a very capable (seaworthy) true sailing machine comparable to anything else out on the market. More fun and more versatile than you can possible imagine, plus you get a nice yak too. You get two boats in one.