There are several models: some with fat wheels for sand and some that have a bit smaller frame than others. Mine is the "full size" with tough tires (foam filled). I recommend it if you can't carry your boat to the water for whatever reason. I have not tried using it on uneven or bumpy terrain so I can not comment on how it would handle that. I bought mine because I was paddling on a lake that you had to walk several kilometres to on a gravel trail.
With wide, soft, floatation tires, the Wheeleez cart makes hauling the boat much, much easier. It is stable, the tires don't dig in, but ride over sand as advertised. The frame is marine grade anodized aluminum tubing, important in a salt water environment. The wheels have actual bearings, which should provide long serviceable life. The poly tires are thin, soft and squishy, which is why they work well over sand and uneven surfaces, but probably could be the weak element in terms of durability. The owner's manual specifically cautions not to over inflate these tires. They are rated for only 2.5-3.0 psi. This low pressure requires a special air pressure gauge, which is not cheap and adds to the total price of an already expensive cart.
This cart is wide and therefore, stable side to side. The cart is low, making for a lot of bending of the back to keep the ends of my 16' long boat from hitting the ground. The low height, coupled with the lightweight tires makes me speculate that this cart would be less than ideal for long portages in the wilderness. That is the down side. For it's durable construction, ease of rolling over soft or uneven terrain, and load carrying stability, give it full points on the up side.
Thus far the maximum distance I have pulled the kayaks is maybe a quarter mile and it was a piece of cake despite in once case having to go over loose sand and another time down and up a steep hill. Before getting these make sure your kayak has a large enough hatch to fit the cart frame if you plan on bring it with you. The wheels come off easily via quick release latches and the wheels can be stored separately. The frame collapses but is still as long as the distance between the wheels. On my longer kayak I place the wheels in the forward hatch and the frame in the back since the frame won't fit in my forward round hatch. My other kayak (the Dirigo) has one rear hatch and it is large enough for both wheels and frame.
The wheels are inflated to very low pressure (1 1/2#) and are designed to pull over sand. The only semi negative is that the quick release latches could be lost so I would make sure you pay attention to storage of them.
As far as durability...mine is holding up fine but I wouldn't expect the tires to last hundreds of miles ;-) They are relatively thin light duty construction. But this makes them light so a good choice for taking along with you on the kayak. If you are planning on dragging the kayak over sharp big rocks you would be better off with hard rubber non-pneumatic wheels....believe they have a version with those wheels also.
Would give a 10 instead of 8 if the frame could be folded in half someway to make it even smaller so it would fit in smaller round hatches.
I was impressed with how well the Wheeleez cart held up after reading so many reviews of kayak carts falling apart. However even though the manual states to use the cart at walking speeds only, I did test it out holding the kayak attached to the golf cart going about 20mph over varied terrain and for some distance.
I have used this cart for over a week now with heavy abuse so I can't comment on its longevity but its held up to some rough use in the last week. The cart also has a extra set of legs that act as a kick stand for when you are loading the boat on and off.
I give the cart an 8 because (1) I am harsh reviewer and (2) I've only used it for a week.