I prefer, conceptually, Bluetooth mice, but in my actual use I've ended up with a dongle-based wireless one. Not having a fiddly dongle sticking out of the laptop is a huge plus.
First, my personal ubermouse (the MX) isn't available in a bluetooth version. So I don't really have a choice if I want that mouse. Which is sad. Sadder still since the USB dongle is huge (sticks out maybe 3cm), and runs SERIOUS risk of getting snapped right off if someone sits down beside me or bumps it.
It does, however, have a rock-solid connection, no sleep issues, and no perceivable lag to me.
In contrast, the BT mouse I own (the RadTech one that can charge its NiMH batteries via a USB cable while you work, which is an awesome feature and also means you don't need to carry a charger while out and about) has an effective range of about 6 inches from my MBP. Seriously--if I have the MBP on a table and use the mouse beside me, it will start skipping. I don't know if this is an issue with that particular mouse (the Wii's controllers are BT, and mine work fine from at least 12 feet from the Wii), with the MBP's antenna, or something else, but it sucks. There's also periodic lag or minor dropouts.
I had an Apple 1-button BT for a while, which had somewhat better range, but it also tended to not wake properly when waking the computer or such.
Interesting side-note: While my BT devices coexist comfortably with my wireless network, if I'm maxing out the 802.11g bandwidth (big file transfer to another computer on the network), my MX will freeze up, so there's obviously some overlap. It's fine under any other network conditions, though.
I've also noticed that full-bandwidth airport transfers will cause the internal speaker in the Wii remote to stop playing sound; all controls work fine, but obviously the way it transmits sound overlaps with 802.11 in some way.