We bought this along with the Maxi-Cosi Pebble carrier for our new baby. At first I was put off by the price, but after more than a year of use I have to admit it has been worth it.
The base is fairly simple to fit, however it also very easy to get it wrong (which might have been what happened to the one-star reviewer). If you get it wrong then it will bleep at you; especially when turning corners, when the weight of the carrier on the unit shifts it enough to trigger the sensors. The way it works is that it has three sets of sensors which tell it/you if it is properly fixed - one on the leg, one on the Isofix struts, and one for the carrier. I had no problems fitting it to the middle seat of my C4 Grand Picasso, but when I moved it to our Ford Fiesta (current model) it took a few goes to get it right; until then I was also greeted with regular beeps. In this case, adjusting the length of the leg solved the problem. We've moved it between cars a number of times, and it's a very simple operation - retract the Isofix struts, remove from car, extend struts again, clip struts into other car seat and push into place (adjusting leg as necessary). Job done.
Once you've got the base properly fitted, then it's a simple matter of dropping the carrier onto the base - the weight of the carrier is generally enough to lock it in place, although you may find you need to wiggle it a bit at first until you get used to it (took me about a week, my wife slightly longer). Again, getting this bit wrong will result in you being bleeped at; a handy display on the base even tells you which bit you've got wrong. Once in place, you can then easily slide the carrier into whichever position you want - lower for a sleeping baby, higher for a wakey baby.
Our baby has now outgrown the Pebble and is now using a Pearl, also on the FamilyFix base. In this case it's less of a benefit as you're not removing the seat as well as the baby any more, but for the first 9-12 months it's definitely worthwhile.