The main advantage I see from adjustable kettlebells is "price", in a sense of the word. I have four kettlestacks (www.kettlestack.com) and they function as a complete set of kettlebells from 10lbs up to >100lbs. Some people will say there is an advantage to progression, and that the large jumps in traditional sizes are a disadvantage, but I don't see that as an issue, for the most part. Varying the resistance by varying the sets reps works just fine.
Price flexibility is the big deal to me. Depending on how I want to work that day, I can adjust the kettlestacks to what I want to do (it's easy, doesn't take much time at all). For example, any given workout I can use (2) 1-pood, and (1) 1.5-pood and (1) 2-pood, or any other combination of the above, or go higher or lower than the above in any combination. Buying the four stacks and all the standard plates set me back around $300, but I have a complete set of kettle bells, 1, 1.5, 2 and >2 KB's. I agree with an earlier post about not throwing adj KB's. That's a disadvantage if and only if you plan to do throwing movements. Also, I can use my std plates that I bought for my kb's for my std dumbells.
As for the type of adj kb's, I looked into many and settled on the kettle stacks as my choice. I called and talked personally to USA Ketlebells and kettlestacks and was just more comfortable with the durability of the stacks, but they were both helpful and very polite.
The only thing I can't comment on is the "feel". That's a bit esoteric to me anyway. I get a great workout with the adjustables, and thats what I want. The only real reason I want to someday purchase dragon doors is to support what I think is a fine institution. But for me, I couldn't afford the full set yet, and I'm sure Pavel would rather have me working out with adj kb's than waiting to buy his, especially since I suppory DD by buying his books and DVD's.
Good luck, you can't really go wrong so long as you start with something.