I am a fitness facility owner, and an experienced personal trainer. I bought this product along with other similar items based on advertised versatility and praising reviews.
This is hopefully the review you need to get the whole story you're looking for; I wish I could have heard what I'm about to share with you...
The other reviews with 4-5 stars must be based on initial impressions/excitement (a flaw in the ratings system overall). This is our experience with the product during the couple weeks of (attempted) use, up to the time we had no choice but to return it. I've been a steady buyer of everything amazon since the late 90s, and I may have made one or two other returns ever.
#1. The individual bags, that are supposed to hold the sand, leak sand steadily. I don't mean one or two of the bags, as if maybe there's a defect- they all leaked sand along every seam of every bag. This is the -seam- of the bag, mind you, not the closure. So I took the idea of contributing reviewers and duct-taped the crap out of it... but it could only hold back the flow of sand for so long. You may be wondering if I must be putting the product through some crazy high-impact routine, but no, it was used basically as a substitute for a bodybar by my clients, a few times, and without any slamming exercises.
#2. I actually UNDERfilled the individual inner bags, yet like other reviewers, we had little success getting all four bags into the outer shell. What makes this product supposedly versatile is ability to change it's weight load. It's time-consuming and impractical, if not impossible, to change the weight during a workout.
#3. The outer shell bag. Take a nice look at the photos provided my the manufacturer. See how the bag looks nice and full, puffy, and solid... well you know how people joke about how fast food burgers look in ads compared with how they look in reality? This is sadly the case here too. Sure, it's a sandbag, but those shots are designed to look deceivingly different than the saggy, lumpy sack which it is. Picture a regular duffel bag with 2 large books in it. A tad more unwieldy than the version the guy in the pic gets to use.
In summary, it's a good idea, but this is a failed prototype- it needs improvement in every area. If SKLZ stands behind their stuff, they will jump on fixing these issues and will probably end up with a good piece of equipment. Right now, you're paying for a heavily flawed beta version.