I switched from a CSK to a SKII to a BB7100t, then an 8700g, because I needed push e-mail for work, and my company runs BES. As already pointed out, the SK series will give you a better chatting experience unless you want to pay an annual fee for VeriChat, and even that may not be as slick as the SK AIM client.
But here's why I'll never go back to the SideKick platform:
- Danger's frequent outages drove me insane (maybe this has been fixed)
- I can install any software, ringtones or wallpapers I want on my BB over the air, needed to jump through crazy "get a developer key" hoops to install custom software on SK
- Google Maps
- Google Talk
- Zen theme gives me the same quick access to apps as [jump]-[letter]
- Legendary BlackBerry durability
I have achieved phone nirvana with my 8700g. I just want to shout from the rooftops that I have found the perfect phone. Glorious screen, great battery performance, blazing fast EDGE, incredibly responsive operating system thanks to the faster CPU. I had fun as a SideKick user, but in my mind there's no comparison.