Not only are they expensive but repairs and parts are high as well. I've heard of people having problems with them leaking. Anytime you have water, which is really heavy, pushing against a door you have problems.
They are really big so getting the water warm enough is another issue. You'll need a really large hot water tank or one that heats large amounts of water quickly.
A better choice, if you have the space, would be a walk in shower with a seat. Make sure you have hand rails and non-slip surface and it will last you a long time. If it's big enough you don't even need doors or curtains.