We had a Hoover SteamVac like the one pictured below, and it was better than nothing, and a necessity for the 30+ years we had dogs. However, it was not as effective as a professional cleaning service, how could it be? BTW, it used hot water, no steam involved really.
If you get one, my advice is pay attention to how much water it leaves behind. Once you've made your cleaning passes with soap solution, you need to go over and over the carpet to suck up water MUCH longer than you may realize - the only way to know is feel the carpet with your hands. You will never get it totally dry, but if you just make one pass, you will leave so much water behind your carpets will smell musty.
Effective carpet cleaning is TWO steps. Getting up the stains and dirt. Then getting all the moisture out of the carpet once is clean(er). Most units DIYers can buy aren't very good at the second step.