I read a couple of archery books by Brian Sorrells not long after I started shooting. I found them to be quite helpful.
In one of them he mentions using a "bow exerciser" to help build back strength. He talks about the need to exercise both shooting right handed and left handed, to build symmetry in the muscles on both sides of the body. I found this to be great advice, so when I use a bow exerciser I exercise both my right hand and left hand forms, even though I actually only shoot right handed.
Brian mentions the "Bowfit" product specifically. However, I ordered one and found it to be very cumbersome; it was difficult to find a way to grip it while at the same time maintaining the correct tube length (although I finally came up with a solution). Additionally, I also found (as some reviewers have stated) that the rubber began to very quickly disintegrate where it passes through the handle, when the tube is set to a length that actually emulates a bow pull. It was also quite expensive for what you get.
So I did some searching and I found a much better product, the Saunders "Power Pull". This is an excellent device, and allows me to work on my form and muscle development while away from my bows. It is a much better exerciser than the Bowfit product:
The blue tube is about 25 pounds, and the green tube 35 pounds. Their site says that both together get you about 70 pound (fuzzy math, I guess)s. At a 28" draw I find that the green tube feels more like 40 or 45 pounds, but that's just my impression. The blue tube likewise feels slightly heavier than stated at my draw length. I find the handle to be very comfortable (considering it's not an actual bow handle).
The only draw back is that the tube angle over your index and ring fingers will be quite severe, with the result that as you draw you will "friction-burn" your skin with repeated usage, eventually creating slight callouses at these points. So I will sometimes use an old shooting tab to protect my fingers.
All told I really like the Power Pull, and can see a noticeable difference from using it. It's especially good for holding at draw to build not just your strength but also endurance. You can just throw it in a carry-on bag or luggage when you travel; it takes up almost no room. It also allows you to concentrate on developing your back form very nicely.
Best wishes,
Frank