In addition to what's been said, I have the Motorola Bravo, and Motoblur can integrate twitter and facebook contacts if when you are starting your phone you say you have the accounts and want to integrate them into your phone. I did not want that, so skipped through those steps. The Motoblur messaging app includes my sms messaging which I always ignore for my dedicated sms messaging app as well as all of my non-gmail emails. This messaging app is not bad, but because I did not put my facebook and twitter accounts in the Motoblur set up thankfully I don't have facebook or twitter in my messaging app.
Motoblur has the options to integrate your social network programs with contacts, messaging, and bringing up what everyone in your circle is doing in Blur widgets but you don't have to use any of those options, and if you don't then they won't bog up or slow down your phone.
If you don't take the integration route then all Motoblur is comprised of is the phone integration, a selection of widgets, and a few extra apps. That being said, I prefer the phone icon and set-up of Blur to HTC's Sense.