Your mileage with Cyanogen Mod (CM) depends on the phone you are running. If you have an HTC phone, you will not have any HTC Sense running or installed at all. So HTC widgets will not work. This is a hurdle for many HTC phone users that enjoy some of those widgets. Personally I've been able to find adequate replacements to them. So that wasn't a barrier for me.
If you are not using an HTC phone there probably won't be a lot of cons. Again though you might not be able to use any stock widgets that came on your phone. But you might have a different dialer and contact app then what you are used to.
Do you connect to an email system at work, such as an Exchange mail system? I do and that was one of the reasons for me not using CM. Not that it didn't work, but there was some functionality I was missing, a few settings and I didn't like the way the contact app managed the Exchange contacts. Whatever HTC uses for their builds worked better IMO so I use an HTC based ROM.
As Saboogu states, hardware will make a difference too, but CM team is good about refining their ROM and working out kinks so that it functions well even on older phones.
If you are already rooted make a NAND backup first. Follow this guide I wrote keeping in mind it was written for an Evo, but most info still applies to other phones. Once you have a backup then wipe and flash and go from there. See if you like it and the feature set. This way if you hate it you can easily restore your backup.