I used the 552 dumbells with my P90X workout and I have to say they made life a heck of a lot easier. It couldn't be easier to select the appropriate weight with these things, and with limited time between excersizes, these were golden. The construction is sturdy; not once did I feel any uneasiness in using them. Even moves where the dumbells were lifted quickly and brought back down didn't cause any issues with the locking mechanism of the dumbells. The only slight issue is the overall length of the dumbells. The width is manageable but the length can cause a few issues in maybe 5% of the excersizes performed. An example of this is doing concentration curls with both arms simultaneously. I ended up having to alternate to get around the length of the dumbell. All in all it is quite easy to work around this small issue and it doesn't impact the 5 star rating for me especially considering that having used the block alternative as well, the bowflex ones caused a lot less issues during excersize movements. 
I used these without the stand, if you do as well make sure you lift with your legs and not your back. If you have a bad back, or just want to be safe, you may want to go with the stand. (Though theres nothing wrong with a little extra leg work with the proper form ;) ) In summary, if your doing P90X, or any other home workout routine that requires you to switch weights quickly, this is the dumbell for you. Also if your going for P90X know that the 552 version covered the weight I needed for every single excersize (I'm 5'11' and 195 lean muscle). I may have been able to go heavier on a few excersizes but the sheer intensity of this workout would have limited my reps significantly. Hope this helps!

By: John W. Gilbert

I did quite a bit of research and had narrowed my choice (like most people I think) down to the power blocks and the selectTechs. Really, the only reason I decided on the selectTechs was due to the fact that they are a Bowflex (Nautilus) product with a great warranty. I had purchased their top of the line bowflex several years ago and it's obvious they take pride in their products. As far as the selectTechs, it's amazing how quickly you can change settings to add or subtract weight. I did read several reviews of people complaining they felt 'flimsy' or some such nonsense. There is a very small amount of play between plates that allows you enough room to quickly remove and then reinsert back into it's holder. Pick up any dumbells at the gym with adjustable weights....they bang all over the place. Anyways, I'd guess if Bowflex had made the fit much tighter you would have these same people complaining they couldn't remove from the holder or that they couldn't quickly place them back into it. My feeling is that they did it about as well as they probably could have. They are very sturdy; I could probably drop the things off of my roof without any wear for the worse. Here's something you might find useful as there are many reviews here with basically the same info. in each. I picked up the optional stand for the selectTechs. Now, it is a great stand...looks great...and is very functional. However, looking back I did not really need it. I would recommend that if you have a home gym and all your equipment is in one area then the stand is a great idea (but you don't need it for the adjustable function of the weights to work...it just sits them up about 3ft). I find myself working out in my living room where I have to transport the weights. I don't move the stand there...it's not necessary (too damn big and cumbersome-but looks great!). So, in short, if you are scrimping $ don't but the stand. If you find yourself working out away from where you store your weights...don't buy the stand. If you are loaded and have a home gym...get the stand. Enjoy....