A tandem duckie will carry either one person, two people, or one person with enough gear to self-support or carry your own load on a trip and still be fun on a daytrip. A single duckie will carry one person and a personal bag - period. Think about what you want it for before you buy.
If you're bargain hunting, see if you can come up with a few hundred bucks more and throw down for something good. Sevylor proves the adage "there's the right way, the wrong way, and the French way." The valves are finnicky but you can make them work if you're careful about getting them screwed in. You'll also be sitting in about two inches of water unless you weigh <100 lbs. I was with a friend with one of the new line of inexpensive Aires and the valves were pretty odd on that one to - I'm not sure how she aired it up but it looked like it needed a special attachment.
When you're shopping, look for D-rings to hook thigh straps to and how you'll be able to seat yourself so you can brace yourself rather than swim. Also consider whether you want a bladder boat, check out how the valve assembly is secured, consider the strength of the fabric, check out the bailing holes, think twice before you buy. You may be able to find a really sturdy one (I'm kicking myself for passing on a Hyside once) at a good deal when rental fleets go on sale in the fall or next spring. That's about all I know. Ask Perry (aka "Will raft 4 beer" aka "the guy who knows a breaching narwhale when he sees one") since he's just gotten a ducky and will probably have some ideas I've missed.
Andy's right, I just bought an AIRE Lynx II from NRS gear swap, and got $350 off and a $100 knife thrown in the deal for buying a 1+1/2 year old boat. It's in great shape. The AIREs are the only IK I know of that will keep my ass above the water, which was important to me because we wnated to use it for cold high alt. lakes as well; all the hysides and sevylors and others ride about 2" under with me, weighing in at 200 lbs. My GF and I just used the tandem for an overnighter recently, and with enough gear for the two of us and food and a pump, the AIRE did fine without sitting below the surface (at 450 lb capacity, it's the highest I know). It also handles well enough the run solo. The AIRE force single IK is supposed to handle far and away the most similar to a hardshell, and can be easily rolled with thigh straps. Normally I would say go with something hypalon, but all my research led me to the AIRE IKs. They are just the best, and the more expensive ones carry the 10 year warranty.
I found the website with the most extensive review of Iks I have seen: http://www.theboatpeople.com/iks.html . Yeah, the shredders look like fun, but they are made out of poor material. I watched one rip an 8" L-shaped tear in it seal launching. Ask stinginrivers about where to find a stinger instead; it has a bit larger tubes than a shredder and is made of PVC (much tougher). As for performance, it depends on what you are looking for: surfing, punching waves, etc. An Ik is probably superior for surfing, but a bit less stable.
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