I've just got a shiny new Lenovo T61p to replace my Dell D820 but it hasn't been an entirely smooth ride so far. I thoroughly recommend reading Keith Combes blog post on installing Vista x64 on the T61p (I've installed Windows Vista Enterprise x64 Edition). If I'd found this before I started I'd have saved myself a few hours of wasted time on the first issue below:

• Using a 2nd hard drive in the Ultrabay adapter - having installed this it worked fine if the machine was booted and then the drive inserted. But booting with the drive installed was a non-started. It took forever and you couldn't see the drive once it was done. See Keith's post for more details and a workaround (add the 2nd drive to the boot order of the machine).
• Black screen when coming out of sleep. This was a weird one. Worked sometimes, didn't work other times. Turns out it was to do with my mouse waking the machine when I disconnected it. I would sleep the machine, close the lid and then disconnect the mouse to move the machine elsewhere. The act of disconnecting the mouse was waking the machine from sleep (with the lid closed) and getting it into a very confused state. Simply switching off the "Allow this device to wake the computer" option for the mouse (in Device Manager) fixed that.
• Startup time seems quite slow compared to the Dell and my "Microsoft-Windows-Diagnostics-Performance/Operational" event log is full of Critical and Error entries relating to startup. In fact it doesn't take that long to get beyond the password screen and start using the machine but for it to *fully* initialise takes an age. In particular I use Start++ a lot but it takes at least 5 mins before it becomes available from a cold boot.
• I had some trouble getting the wireless to connect to my router using WPA security (I decided to dispense with WEP having read reports of how insecure it is). In the end swapping my router for a later model resolved this (but it's odd as I've not had that trouble with any other machine, XP or Vista).

Aside from that I'm very happy with the machine. Performance is great, it's very stable and it feels more solid than the Dell. Battery performance is excellent and it also has a built-in multi-format card reader and even a keyboard light for those awkward moments taking notes at the back of a darkened conference.