Gnome is a window manager. It's one of the more popular ones. Generally its counterpart is said to be KDE, though there's a lot of good, free window managers out there, and it's really all up to taste as to which one you use, since the fine particulars of them all run pretty different from one another.

I use it off and on - occasionally there's some functionality missing here or there that would seem small, but it drives me crazy Really, you shouldn't go into it expecting a free version of Windows. Everything is done significantly different between the two. Like, you can't natively run windows programs - it can be done with Wine, though the results of that can vary from "perfect" to "totally borked." But there are a bunch of natively made apps to replace the functionality of Windows programs that generally do it quicker and better.

I guess here's some pros and cons.
Pros:
- Completely free, down to the source. You won't ever have to worry about a $200 license for a Linux distro (well, some are commercial, but they always offer free versions). Generally all of the software available for it is free, as well, which is a really significant thing - you can get professional level applications completely free, with source.
- Rock solid. You can leave Linux up and running for months with no problems - it's just that effecient at managing resources, unlike Windows, which makes your system bog way down. It just doesn't happen on Linux.
- Secure. You won't get viruses or spyware on it - most of those are written to target the normal, unsuspecting Windows user. Frequent updates to various pieces of software in your system make sure you stay up to date with all of the recent security patches.
- Powerful and versatile. A Linux box can be whatever you want it to be, from gaming rig to office computer to server technology to an educational pc, and on and on and on.
- It's fun. It really gives you an opportunity to own your OS and do whatever you like to it, the process of which is educational and entertaining. You'll learn a lot about computers if you really get into Linux.

Cons:
- It's not Windows. Some would say it's a pro, but if you're absolutely set in your ways of how you like your OS (Windows-like), you'll probably be disappointed.
- Hardware support can be limited. Actually, most of the time, it's not. Printers and wireless network adapters, though, tend to cause problems, which required a relatively hackish solution.
- It can be hard. If you want to use Linux, you have to get used to the idea of constantly having a command-line terminal open to really get into things - a lot of newer distros are implimenting GUI controls to make it easier on new people, but the fact of the matter is that if you want to do anything significant on your system, you'll be trudging through the command line. Also, you'll have to do a lot of your own footwork. A lot of times when you go to boards asking for help on [x] small problem with your OS, they'll tell you to google it, because it's something that's likely been asked many times before. So get used to doing lengthy, thorough google searches and searches of your distro's message board before asking for help.

So there you go. Really, it's a wonderful experience, even if you just do it as a side project. I recommend Ubuntu Linux, just because it makes things really easy for a first-time Linux user.