I've been JB for nearly every day since the technology existed. I've had some scary moments, but the process is very safe now if you just follow directions. The benefits FAR outweigh the cons.
The biggest con is the fact that you get addicted to the freedom and it takes some work to get back all that functionality after a firmware update.
Though the process is easy, don't oversimplify what your'e doing to your phone. Too many people jump into this without doing their homework. They tweak their themes, install a bunch of hacks, then as soon as new firmware shows up, they jump in and do an update and get stuck somewhere in the middle of the process. They then start crying BRICK (which appears to be extremely rare, if it really even happens at all) and whining about how much they regret ever jailbreaking their phone.
If you decide to JB, do yourself a favor and understand the nature of what you're doing so you can undo the right things before you upgrade your firmware. That's part of the cost you pay for freedom. Don't be lazy or you're asking for some troubles... but as has been stated, you can probably overcome them with some research, because you won't be the only one.
Oh yeah... and don't be the first one to upgrade your firmware when an update comes out. Wait for the pros to go before you and identify problems and solutions first.
Just words to the wise...