They're basically a big chunk of rubber used as a suspension stop. I have them on my '98 and they work great for my needs. My brother had them on his '03 and hated them for the ride quality. His problem was his springs had lost some arch due to a lot of heavy loads prior to install. My dad now has them on his '04 and it rides normal. There's about 2 inches of gap if i remember right so unless you hit a huge bump, you won't know they're there. For the price, i love them.
Had a set on a Safari Van I had…they are hollow rubber stoppers that take place of the stock stoppers on the rear end. They come in contact with the frame when you put a load on and support the suspension. The only thing I found that empty when they hit they will bounce the frame back up. That can be a nasty bump.
On my truck I went to air ride so I can regulate the ride…..
I just put airbags on mine ( a month or more ago), with an onboard air system and auto leveling valve. Makes it much easier for my wife when she takes the camper somewhere. Cost me about the same as one of the cheaper single switch onboard air systems, to get the parts and put it together myself.
I had timbrens on another truck, it was kind of a bandaid for worn out springs. I also have something similar on my plow truck, which has too much spring. They are only 1.5" from the axle and I'm not sure they ever touch it. There was still 1/2" of clearance when I had over 2 tons in the bed.
Timbrens, as stated, are a good inexpensive way to increase your occasional load capacity. They aren't (IMO) a suitable fix for heavy towing or "squat" problems, as your ride quality will suffer.
Air bags, being adjustable, are superior.
That said, I have Timrens on my HD work truck and am pleased, but it has ultra HD springs so very seldom bottoms out.