I used the Bowflex 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!