I've set up one in my shower an it's a great way to douche the rectum and lower sigmoid in preparation for play or for rinsing out afterwards, but I wouldn't use it for much of an enema simply because there's no way to control the volume of water or the pressure, only the flow rate.
Some considerations...
Almost all drain plumbing consist of 2 systems, a large diameter "soil" system which drains the toilets, and a much smaller capacity "waste" system which drains sinks, baths, and washing machines. (The waste system connects to the soil system before leaving your house.) Your shower will be connected to the wastewater system which was only designed for water, dirt, and often gets clogged with hair over time. What you wash out of your butt will be going there, so think about how you're going to make that work before investing. Unless you make major plumbing modifications, it's not going to be able to handle much. That's why I think of it more as a convenient douching system than an enema system.
Another consideration is what are you going to tell children or visitors who need to use your bathroom and spy this thing hanging from the shower head? (Me, I remove mine and stash it in the cupboard in advance.)
I wound up improving on my system by replacing the combination control valve/diverter that it included with a simple on/off diverter and added a separate flow control valve after it. That way I can simply click the diverter on/off to engage and already have the flow set at a fixed rate. I also wound up replacing the included 6' hose with an 8' one so it could be used in other than a standing position. And lastly, reviewers of various vendors report that some of the metal nozzles are only plated and deteriorate with use. (As it happened, mine included a solid aluminum nozzle). If I were to do it all over again I'd probably put it all together from scratch and avoid a kit since it would've cost less, but if you're into anal play it's definitely worth having. No need to mess with the bag for a quick lower bowel rinse.