We have a bath lift for DH; it is operated by a rechargeable battery and means he can enjoy a bath safely, carefully supervised by me! I think ours cost around £300 and has been money well spent. It lives permanently in his bath but lifts out for cleaning. We were advised not to go for the inflatable type so I have no experience of them.
The downside is that you need to put quite a lot of water in the bath in order to immerse the posterior properly, but otherwise we have no fault to find. Have had ours for some three years now.
There is one, that is just a pull out type of thing, and it lowers you (and raises at end of bath). As it is so thin no need to have extra water, and it also goes into a wall holder when not in use, meaning the bath is completely as normal for any other users.
I've seen this one in a number of homes - it's very good, doesn't need any plumbing and easily removed for any bather who don't need it. It also has back support, unlike the pull-out thingy mentioned above.