Is this a good SSD for a OS

Any SSD will be faster than a traditional hard drive. Sandisk is good. Samsung / Intel are better but generally more expensive.

I have a older Kingston (V200+) that smoked my Seagate 250gb (single platter). XP boot time when from @ 1 minute to < 20 seconds.
 
Yes. Any SSD will be a lot better and faster than a traditional hard drive, not to mention a 5400RPM one. More expensive SSDs usually have more features and they are more reliable but a mainstream SSD already has a very little chance of failing as they are not prone to shock / dropping.
 
What are you using this SSD for? If I read correctly, you are going to run your computer off of it, and maybe you will use this as your only drive. In that case 120GB is kinda small. I would suggest looking at the 240~256gb SSDs.

Also, at this point, be wary of the performance upsellers around here. As many have said, any SSD will be quick and will do you well. However, you will always have performance/brand fanboys who will try to get you to buy whatever Samsung/Intel/OCZ/blah that costs "just a little more" for "massively better blah". Maybe do that later. Not now.
 
Go with the Samsung 850 EVO. It has a longer warranty and it is more reliable with better features such as some Data Loss Protection software which I forgot the name of. I'm also using one in my own system so I can vouch for it's reliability! 😀