After a lot of consideration, I ended up purchasing these weights to use with P90X Extreme Home Fitness Workout Program - 13 DVDs, Nutrition Guide, Exercise Planner. The program calls for a lot of different weights, and I was tired of leaving 8 different dumbbells lying around on the floor of my den. After using these for roughly three months, I am very pleased with them, and with a few exceptions, they work great for P90X.CONSTRUCTION/DESIGN- As of 2/16/10, the model being sold on Amazon is the 2009 model, which came out in September 2008 and is the most current model available. There is no 2010 model, despite what many third party sellers claim (I have confirmed this with Nautilus). I have uploaded pictures of the version Amazon is selling, which comes with a black base plate. Amazon has lumped the reviews from the 2008 model in with this 2009 model, so read the reviews carefully. The 2009 model eliminates many of the complaints of the 2008, most specifically, it uses metal tabs on the weight plates to lock them in place. Construction is solid and it is very unlikely these will break. Although unlike traditional dumbbells, you can't drop these on the ground after a set. They are fairly sturdy, but I would be very nervous about dropping these from any height over about 5". The handle is ergonomic to fit the palm of your hand and the grip feels good and natural during lifting. The handle is thicker than a dumbbell and I prefer it to the thinner dumbbell handles.The weights have a dial on the end of them that you turn to select the desired weight. If you select 10lbs, it will then lock in the two weight plates to make 10 lbs, and release everything else; so when you pick up the weights, you get 10 lbs and all the other weights are left in the base plate. When done, you put the weights back in the base plate. This works very well, with the exception of it "catching" occasionally if the weight plates shift a little bit. This happens maybe 1/100 times or so though and isn't a big problem. You just jiggle the weights a bit until it locks. The nice thing about this design, is that the two ends of the dumbbell don't have to be the same weight. You can set the dial to 10lbs on one end and 30 lbs on the other. This will effectively put 5lbs on one end of the bar and 15lbs on the other. I don't do this often, but it can lead to more variation in exercises, and I occasionally do it for tricep workouts or hammer curls. When lifting, the weights feel very sturdy. The plates are firmly locked onto the handle. There is no shaking or movement of the plates at all, and they don't feel much different than dumbbells.PRICE - I definitely do not think these are worth the suggested retail price, but then again, I have never seen them sold for anything close to that. While they may seem expensive, if you calculate the cost of a large range of dumbbells (which typically sell for $1 per pound), these are a very favorable comparison. Just two 35lb dumbbells are going to cost you at least $50 (closer to $70 if new).USE WITH P90X - I originally did P90X with dumbbells. I had three different dumbbells, which just isn't enough for the variety of exercises used in that program. If money and space is no object, I would recommend a full dumbbell set over these, as dumbbells are better for the exercises. But for most people that just isn't practical. I hate pulling out and storing even three sets of dumbbells every other day to workout. The SelectTechs look so good that I just leave them in my den year round.The main advantages the SelectTechs have with P90X is the ability to select smaller weight increment, and being able to make weight changes quickly. Before these I owned a set of 10s, 15s, and 25s, which isn't enough variety. Going up from 15 to 25 on bicep curls is a very big jump for me was too light, 25 was too heavy, and my form was terrible because I was struggling with the weight. With the SelectTechs, I could go to 17.5, then 20, then 22.5, then finally to 25. being able to go from 10lbs to 12.5 on tricep exercises, rather than 10lbs to 15 helps significantly. The other advantage is to quickly change weight increments. With P90X, you have about 15-30 seconds to set up for the next exercise. 5-10 seconds is all the time I needed to change the four dials on the SelectTechs and get a new weight.CONS - The real problem with these weights is the length. I had no real problem with the width, which is very manageable, but the dumbbells are 16" long. This can cause a problem with some of the P90X exercises. There are maybe 7-8 exercises during the entire program that are awkward to do because of the length of the dumbbells. There are about 2-3 exercises that are very awkward to do (to the point that I actually switch to my dumbbells for these). Congdon Curls and Flip Grip Twist Kickback (tricep exercise) are two that I can think of off-hand that are difficult to do. You can ultimately make it work, but it's going to feel a little funny as you try to hold the weights out far enough to clear each other or your chest. This isn't enough of an issue that it would discourage me from buying them again.I originally debated between getting these and the Bowflex SelectTech 1090 Single Dumbbell, since I was worried 52.5 pounds wouldn't be enough. Even though there are a few exercises where you could eventually need more than 52.5 pounds (heavy pants and lawnmowers come to mind), I really think this is enough weight for 90% of the people who will be doing P90X. And as Tony says, if you don't have heavy enough weight, you can always add reps. I would not buy the 1090s (which go up to 90 lbs) for P90X because I think the length of those, which is even bigger than the 552s), would just be too unwieldy for many of the exercises.The SelectTechs did come with a short DVD that shows you some workouts specifically designed for these dumbbells and how to use them. I did find this beneficial since it gave me some tips that I didn't know about lifting. Optionally, I bought the Bowflex SelectTech 552 / 1090 Dumbbell Stand to use with these. It looks great, but I'm not sure I would buy it again. If you don't care about looks, the box the SelectTechs are shipped in makes a great stand on it's own and is very sturdy.UPDATE June 2012 - I've put these weights through an estimated 200+ hours of heavy use since I've had them and they still look brand new and work just the same as when I got them. There are times when I want heavier weights and would have liked some dumbbells that go up to 90 lbs, but I don't want to sacrifice the smaller profile of these for the larger model. Still very satisfied.UPDATE November 2013 - I'm still using the SelectTechs almost daily. They are in almost identical condition to when I received them with no signs of wear or problems. I have not found a better solution for adjustable dumbbells yet and will continue to use them (although I do wish they were slightly smaller).UPDATE November 2016 - Still using the SelectTechs, albeit about bi-weekly since I have switched my fitness regimen. I would say they have close to 1000 hours of use and are still in almost brand new condition, and still working great. I have recently switched to the Selecttech 560s though and will be finally retiring the 552s.