Review (4/5): A good alternative to their inexepensive brushes
I've been using this for about four months. I think it's better than the Sonicare I tried, but which you prefer is going to be an individual choice.
I've used manual toothbrushes for most of my life, but about nine months ago my dental hygenist prompted me to try an electric as a potential time-saver.
I first went with an inexpensive Oral-B which ran on AA batteries, and found it was a bit easier to use than a manual. The small round brusheads are easier to fit behind my back teeth, and electric toothbrushes in general seem to stimulate the gums a bit more. The big advantage for me is that it's faster; I used to brush for 3-4 minutes with a manual, and I find 2 minutes with an Oral-B seems to work just as well.
I didn't find it was earth-shakingly different, though. Since my dental hygiene before switching was "near-perfect" (I get checkups every three months and only rarely does my hygenist have to do anything at all) neither I nor my dentist have noticed any differences. I'm not convinced that electric brushes in general are more effective than a properly-used manual.
The cheap Oral-B brushes are... really cheap, and I think they're overpriced given how often they fail. If you want to spend $50-$100 a year replacing toothbrushes, that's certainly one way to do it. I tried two of them, and both of them failed in different ways within 3 months.
I next tried a top-of-the-line Sonicare; a local store offered a 30-day money-back guarantee so I figured it was worth a try (they don't sell any non-cheap Oral-B products or I would've done that instead). Yes, it was probably built well... and heavy, and the handle was immense compared to the Oral-B. I found it very awkward to use, and I don't think it was doing a very good job of cleaning because I noticed substantially more gunk from flossing than before I started using it.
I don't mean to complain about the Sonicare or claim it's a bad product. My point is that there *are* differences, and you may very well find one electric works better than another. My hygenist mentioned that some people greatly prefer the Sonicare brushes, and they can't be that bad all around because many people are successfully using them. They just didn't work for me (more accurately, I should say the one I tried didn't work).
The 8850 is built quite a bit better than the cheap Oral-B brushes and it should definitely last longer. It's certainly more waterproof (I had problems with water getting into the housing of the others). The size fits my hand well, and it seems just as easy to use as their others. I also like the variable-speed feature, though I wish there were a way to set it once rather than having to reset it every time I use it.
Plan on securing the base to a wall. If it's sitting on a sink or shelf it'll probably need cleaning more often, and it's pretty easy to knock over. Also note that the tongue cleaner or vibrating pick attachment don't quite fit in the attachment holder; it's apparently meant for brushes only, which could be a minor irritation if you use them regularly.
The timer feature might be useful, but that depends greatly on how you brush. I don't use quadrants, instead I brush my lower teeth (doing the fronts, then the tops, then the backs), then the uppers. Thus the 30 second timer doesn't help me very much, and I could never quite figure out what the 2-minute timer was for--what happens when the two minutes are up and I haven't finished the backs of my upper teeth yet?
I've never needed to use anything other than the ultrasoft heads, and since I usually brush 4-5 times a day I prefer using the most gentle brush I can find. (The general advice is that stiffer brushes don't clean any better anyway.) The other attachments (tongue scraper, vibrating pick) seem OK, but I prefer using floss, a manual gum stimulator, and a plain tongue scraper instead.
The pick replacements are very expensive and, in my opinion, just aren't a viable alternative to floss. Some people prefer using a vibrating tongue scraper because they don't stimulate the gag reflex as much.
The Oral-B head replacements in general are expensive, and they really do need replacement every three months (mine start visibly fraying around that time). It's worth taking that into account when looking at prices, because they're a substantial investment. And shop around, occasionally they can be found for much less than the usual retail prices.
If you haven't used an electric (or an Oral-B) before, I'd suggest trying one of the cheap/disposable electric brushes first... and you may want to try two or three different brands so you can get a feel for which style you prefer. Money-back guarantees are important here, because some people seem to really dislike certain types of brushes.
If you've used and like the cheap Oral-Bs and want something that will last for more than a month, the 8850 would be a good alternative. I'm not convinced the uber-3D cleaning action (or whatever they're calling it this week) really works any better than their others... but it probably doesn't hurt, and the two-year warranty on the 8850 is a win.