I bought one of these back in May, and have two brews in with it.
I bought it as a "toy"... something to keep the fun in brewing. I really liked the idea of being able to see how my beer was doing, without sampling every day. (especially since I am often out of town for a week or so hence the only two brews)
I found it very easy to setup, and pretty accurate in indicating how the brew was fermenting. Cleaning has not been a problem between batches either.
Now, while I said I bought it as a toy, I have found it to be a pretty good tool as well. My last batch I was brewing... had some issues in the process:
1.) I had pitched three vials into a lager (first time not doing a starter)... and waited and waited.... 5 days later while out on a fishing trip, I opened up my phone and looked at the readings and I announced that it was actively fermenting!... ( I never have lag times that long and was preparing myself for dumping my first batch ever... me thinks I will always do a starter from now on...)
2.) I was able to track the drop in gravity and properly allow the temperature of the fermentation to rise to help the yeast finish out.
3.) A week into lagering, where i usually drop the temps a degree or two each day... I saw a temperature spike on my phone about three days in... (my son had unplugged the fridge accidently) I caught the issue within 3 hours of it happening... so no real harm done.
4) Most of all, I really like being able to call up the graph a couple times a day during a hecktic work week and have a few moments of zen.
In my experience, you do see a spike in OG just as fermentation kicks off. (I like it actually because it is a great indicator... it falls back to normal within a few hours) The final gravity is not lab precise... My lager finished 1.008, and the bug was measuring 1.012... but that is not really an issue to me. The big thing is the ability to monitor the fermenetation activity from anywhere, and the trend data really comes in useful in deciding when to allow the fermentation temps to rise up.
All in all, there are more important things to spend your money on... kettles, yeast starter equipment, fermentation chamber... but I would recommend this long before the digital refractometer, PID temp controlled burner, and conical fermenter.
Personally, I found it money well spent and would recommend it to someone like me who is a bit OCD with their fermentation. (I failed "pragmatic brewing 101" years ago