Better than lead competition.
I'd like to start this review with a disclaimer. This is coming from someone who is mid P90X and has made great progress. I built up from regular screw on dumbbells to these. I'm not a body builder and I'm a beginner.
I originally looked at a few different weight sets. While looking I considered the following factors: Durability, convenience, versatility and space saving.
Durability: The majority of this weight set is made out of metal, mostly the important parts: the weights and weight adjustment part. The handle is a durable solid plastic and the other plastic parts are mostly just for show. Overall this weight set is durable and sturdy. This is a primary reason I selected this set, it seems like it'd be able to stand the test of time.
Convenience: The weight set comes with it's own cradle. The cradle is just plastic, but it works. The real nice part is the easy adjustment. It's just a quick turn of a handle and slide to the weight you want and slide back. Super simple. Doing P90X I need to go sometimes from 20's to 50's then back to 20's quickly. Simple place it on the cradle, move the slide and off you go. Great feature.
Versatility: Here is unfortunately where it comes up a little short. The max weight for each dumbbell is 50 LBS and it only moves in 10 LBS increments. So making the jump from 20's to 30's can be really rough, if it had 5 LBS adjustments instead of 10's it'd be better for lifting, but you can just simply add more reps until you're ready for the 10 so overall it's not a bad deal.
Space saving: As mentioned above I needed 3 adjustable sets and they still couldn't do all the weights this one could. I'd need a ton of space if I were to use traditional hex weights or even adjustables. Great feature.
Now with all the great things being said, time to move into my biggest complaint.
Although the design is neat looking, they are a bit bulky. The one issue comes is sometimes I must adjust my body positioning as not to hit myself with the weights as they are extremely wide (in order to fit up all the adjusted weights. When you compare it to a dumbbell loaded with 50 LBS though, the bulk isn't too much more. Overall, while it sometimes gets in the way it's still a great set.
Final recommendation: I would highly recommend this weight set over the higher priced and less durable Bowflex. I haven't tried others but I've read enough about bowflex to know they're build not to last. These weights are a good set. They are worth the money!
Update: 1/8/11: The weights are still going strong with no signs of slowing down. I use them three times a week for about an hour, so they get plenty of wear and tear.
Update: 4/26/11: The weights are still in excellent shape. No oil, no upkeep, just solid workin' out.
Update: 3/1/14: The cradle for one of the weights has chipped from usage and the weights will sometimes fall over or off the cradle. The company wants $25 (with shipping) for one replacement or $50 (with shipping) for two replacement. A little hefty of a price, especially since this review helped sell a bunch of them over the years. It's still a good product and I have no complaints other than the cradle finally chipping and the hefty replacement fee. I'll see if I can find a way to fix it myself before spending $25 on a replacement. Official Amazon review of XMark Pair of Adjustable 50 lb. Dumbbells. Click for additional reviews.