I do them 3x8 after Squat/Press/PC days, I wouldn't be able to recommend them either way for back rehab. If you're rehabbing your back but still doing a regular SS type workout I'd leave them out. However if you're not able to pull/squat I THINK they'd be a good addition to a workout. I wouldn't drop crazy cash (what's crazy cash) on the thing, but $200 or so probably wouldn't be a terrible investment. A roman chair I think it's called, is also great for ab work.
If you have welding/metalworking skills, I'm sure you could build yourself one pretty cheaply and it'd be more solid than most of the commercial setups you'd be able to afford.