I think it's worth trying at least. As others have said, Ubuntu comes on a LiveCD. That means you can boot up from the CD and use it like a normal OS. You can browse the web, mess with settings, even install software. It's a great way to try it out. Then if you like it you can install it onto your hard drive. Ubuntu is no longer officially supported on PowerPC systems like your iBook, but it is still available
here.
I've used a lot of Linux distros, including several versions of Ubuntu. It's hard to give a meaningful answer from anyone on how Linux compares to OS X or Windows, because everyone's needs and habits are different and I'm probably not what you'd call an "average" user. Most of the time I've spent in Linux has been fixing problems, but then I also experiment a LOT and have advanced needs, so maybe that's not so strange.
There are a lot of great things about Linux. You have an excellent selection of free software that works very well on Linux. OpenOffice on Linux is MUCH better than OpenOffice on OS X, for example. On OS X it's painfully slow (don't ask me why), but on Linux it's smooth and fast. I guess this shouldn't come as too great a surprise since most free open-source software is made for Linux first and foremost and then PORTED to other platforms. Most open-source software does not run as well on OS X as it does on Linux. That includes OpenOffice, GIMP, and even Firefox (IMHO).
Linux still has a lot of things that are hit-or-miss, though. Simple things that Mac (and even Windows) users take for granted are not always simple in Linux. For instance, I've tried lots of different distros, and each one seems to fail in a different way when it comes to sound. Sound is something that I've always just taken for granted as a Mac user -- it Just Works™ -- but on Linux, just getting a global volume control is an issue. It can be maddening. On the other hand, plenty of people seem to have no issues with it at all. Again, it's hit-or-miss. Some people seem to be unlucky. Mounting disks (and especially disk images) is also sometimes a trial in Linux, although I've personally found Ubuntu to be pretty good about it.
As for which Linux distro you should use. Ubuntu is a fairly good one. I recommend using their "LTS" (long term support) version, which tends to be more stable, rather than the latest version. Personally I like Debian (which is what Ubuntu is based on). The two are very similar, but I've found that Debian has the fewest glitches for me for whatever reason. I still recommend Ubuntu to other people a lot, though, because it's more newbie-oriented.