I've had my Peragon cover for several month's now,and really like it.While it isn't water tight,and very few bed covers are,it keeps the bed interior pretty dry.
I have a tailgate seal,but haven't installed it yet.Most of the water that gets into the bed comes from the gaps around the tailgate.
I have an Armor Lid 1 piece bed cover on my other truck,and it gets water inside the bed also,but it also doesn't have a tailgate seal.
There are only 2 negatives that I can find in the Peragon bed cover.
1. It gets very,very hot in the sunlight.You need gloves on in order to push it down to open it,or you will get burnt hands/fingers.I know this from experience.
2.If you have any small items-boxes/coolers/etc.,and don't use a cargo management bar,and those items happen to slide to the front of the bed.You will not be able to open the cover to get access to them,you will have to climb inside the bed,under the cover to retrieve them.I also know this from experience.
I now have/use a cargo bar for all small items that I carry in the bed of the truck.