I got the Retrax One because a buddy has it on a 2500 Chevy Duramax and he loves it. The main benefits for me are the ability to securely lock stuff inside the truck bed (along with a locking tailgate) and waterproof transport of gear in the bed.
Locking: I frequently ride around with a fishing rod holder, roto molded cooler and/or metal gas can in the bed of the truck. Instead of worrying about using a bike cable and two padlocks to lock all that stuff to a tie-down point - I just lock the tailgate and Retrax. The gear in the bed is initially out-of-view for casual thieves who might eyeball whatever gear was left there. I don't know about long-term storage of super expensive gear in there but for opportunity thieves - the locks are sufficient to keep them away if they do decide to spend a minute making an attempt to get into the bed while you're in a store/restaurant or something.
Water: I live where it rains. A lot. Four guys can use the cab of the truck to go on a quail trip and four duffel bags + gun cases can ride in the bed out of the rain using the Retrax. No more luggage taking up room on the back seat or in the back floorboard. I have only had mine for a few weeks but I haven't seen any water get through it. Even after a drive through car wash.
This wasn't a big consideration for me because my truck came with bed liner from the factory but: I think with the Retrax you could get away with a metal bed floor, rubber mat and plastic toolbox. You could spend the $$$ on Retrax and save on the spray-in bed liner and metal, locking toolbox. Depends on how you want to set it up but it's an option for sure. For now I'm riding around with the Husky plastic tool chest - I could probably get away with it for the life of the truck. I suspect I'll eventually get a welded, metal toolbox . If my truck had come with a bare metal bed I would have dropped in a rubber mat, the Husky and gone from there.
I don't know what you mean by t slot accessories but if it's a Toyota reference - I haven't tried to use mine yet. They're still in the packaging. Will they work? I thought they wouldn't. At least with the Retrax One.