There are certain apps in the market that are root only, so that might be a consideration for you to try rooting. Some of the ROM's work to minimize the so-called 'extras' that come pre-loaded on your phone, so more than likely you won't see carrier-branded apps after rooting. This may allow the phone to run smoother or faster and possibly free up resources to put apps of your choosing on your phone. Rooting really opens up your phone to customize it as much, or as little as you choose. If you are interested in over or under clocking, you would be able to do some tweaks for that if you root.
On the con side, really it comes down to how tech savvy you are and how well you can follow the instructions the developer laid out for the process. Over on xda-developers there are at least two methods for rooting the Inspire. I believe someone posted a thread here also that mentioned a third-party outfit that will root your phone for you (for a fee). Also, if you are of the personality to always follow the rules, you have to know that rooting your phone will void your warranty (if your carrier finds out). There is a potential to brick the phone in the process as well.
The Inspire is my first Android phone, my previous phone was a Blackberry and in my opinion the BB was somewhat easier to switch OS than this phone. I'm sure others will have their opinions as well. Good luck with your decision! I think it's a pretty fine phone either way.
CM7 refers to Cyanogen Mod 7, one of the ROM's available. The standard is the most stable, there are also nightlies that are less stable and considered to be in development so they are subject to wigging out I guess.