I bought this to replace an aging Extang Revolution. I loved that cover mostly because I don't like covers that have velcro to seal the sides.
This cover does not use velcro. I love the very low profile and the ease of rolling. I also like the easy close at the tailgate. The Extang had springs and needed to be stretched to close it. The mechanism eventually fails and it becomes hard if not impossible to get it closed and latched.

I really like the weather resistance of the X4S. So far I've only seen small amounts of water in the truck bed after rain storms.

The negatives on the X4S, at least for me, are 1) the knobs that hold the head of the cover at the front of the bed have nylock nuts in them and they are VERY hard to tighten up, despite the knobs. I may change them to standard nuts and lock washers. 2) The hold open straps now have this sort of T connector that slips into a holder and is supposed to stay put once you tighten the strap. So far, I've seen the straps loosen on there own and the T connector is in danger of falling out. I'm trying to figure out a better method of securing the cover when it's open. But some have said this in an improvement over the X4, but I don't really buy that.

Vinyl flap near cab about 1+" short of being flush with bed (69") per instructions.

My initial install on my 2005 Silverado with the wide, 6'6" bed went pretty quickly. The instructions were easy to follow and all of the parts worked as described. However, when I got done, the bed cover itself wasn't wide enough to latch on both sides. It was missing by about 1/4". So I contacted BAK customer support, described the issue, and sent them pictures. They sent me a set of shims. I took the rails off, applied the additional shims, and put the rails back on. This time, the rails were the right width for the bed cover to latch securely on both sides. Kudos to BAK customer support for their quick help making this cover work as it is designed.

On that note, it works great. It latches securely (now), looks great, is fairly waterproof, rolls up tightly against the back window, and rolls back down easily. I had a folding cover (BAK) on this truck for 15+ years and much prefer this design.

Quite pleased with fit and function also how Quiet it rides!

Absolutely love it except for the plastic end caps at the tail gate. They could be attached better with a screw and washer. one was broke when I purchased it . But was taken care of promptly by there excellent customer service.