I've been playing with this now for a bit, through Beta 2 and 3, then to GM and I have to say I was disappointed in the install, but happy-ish with the end result.
I have been using it on a clean build on an external drive and so had not experienced any of the issues involved in an upgrade from a well embedded build of Snow Leopard.
I had two MacBookPros to upgrade.
The first is my work machine mainly used with Productivity apps and some semi- and Pro-applications. This does not store any mail, does not store significants amounts of media (video / audio), has a number of encrypted files (TrueCrypt) and a number of VMs on Parallels, VMWare and VirtualBox. This one went through fine apart from TrueCrypt not being happy finding and mounting some of the volumes, Parallels needing a re-install and one of the iPhoto libraries (I have several to swap between) becoming corrupt on first opening.
I then went for an upgrade on my Home machine. This is also an MBP (now 4 years old) but going strong. It has a largish amount of directly attached storage, for files, for iTunes, for Photos (iPhoto and Aperture), for iMovie and for email ... a heck of a lot of email. I now run across 9 different email accounts, partly due to legacy reasons, but I download all my mail and attachments. I have done this for the last 12 1/2 years and this is reflected in the size of the mail store I have, including having been a member and archivist for a number of mailing lists.
And so we hit problem number one. I leave external devices attached as there are soft links to some files and folders in place. I do turn off certain things, like AV, etc ... and I did turn off Time Machine too. Unfortunately ... and I don't know why ... Time Machine turned itself back on ... whilst the files were being written and it stopped the restart, crashing out the installer, charting out the backup and hanging the drive. Eventually there was a Kernel Panic. On reboot the internal HD and the HD of the Time Machine device (a Western Digital My Book Studio) were corrupted. The Lion Recovery partition could not fix the Internal volume and threw a fit when trying to talk to the corrupted external drive. I had to use the Snow Leopard Disc to repair the internal volume so it would boot ... but would not proceed with another attempt to upgrade. To repair the WD drive I had to plug it into my MacMini and it took several attempts to get the volume usable. I copied files off onto other drives but I lost my Time Machine snapshots ... which had been running fine for about 6 months (since I had to go to a larger volume).
I took a TM snapshot onto another external drive, did a wipe of the local drive, installed SL and then ran the upgrade. It didn't like plugging in a TM volume (yet my work one did and was happy to restore files) so I did a fair bit of manual file transfer ... and then I hit the problem of Mail. Apple Mail is improved in many way ... but I liked it as it was, which is why I had kept using it rather than using whatever Mail/calendar/etc app is part of Office for Mac at the time. The upgrade of mail took some time to run (well, the collected Mail folder I transferred was 12.5 gig ... so I expected it). the things that have not worked were the Mail rules, the application of mail settings for accounts (even though I told it to sync the details with MobileMe ... but it looks like the mail setting .. or lack of ... on the work MBP cleared them out!), and then we hit the published problem of it not liking how some Mail providers do POP. It doesn't like 2 of my accounts so I have to use webmail for them for the moment, or use another mail provider to pull in first ... even though I want to keep them separate (long story, but comes down to audit trails mainly).
And then I find that some NAS and external drives need more updates to work ... Most of mine are WD and the external drives are fine (apart from the knackered Time Machine which has had to have a complete wipe) but the WD NAS I have is not happy and the connection drops out when trying to do any file transfer longer taking longer than 5 mins (which is anything over 200 meg since there is a lot of wireless interference atm and it is causing issues ...)
So, the things I have yet to do to keep things going ...
1 - Recreate all my Mail rules ... I had over 100 and it helped me manage my email life.
2 - Work out why Safari does not always go back to the previous window on the old MBP but is fine on the new one. I have a feeling it is down to the trackpad navigation but will play over the weekend.
3 - See if there is a way to have the Launchpad on *both* screens. I have 2 screens and run extended desktop as I like *large* screens. a 17" MBP is nice, but having a 27" monitor is better. That is the primary monitor and the MBP screen is secondary. That does not mean I don't want to use the secondary monitor ... I do, especially at work (13" MBP and 27" monitor with the monitor as the secondary display), and want to use both screens ... call me an iOS heretic, but 2xscreens=good!!!!!
4 - Work out why, on the work machine, after encrypting the full drive, then putting the BootCamp partition in, installing Win7 64-bit does it work fine until I install the Bootcamp tools ... don't have the problem when the drive is not encrypted ... but I get BSoD about 1 min after boot up and cannot find why.
5 - Spend time to get a different Time Machine backup option, possibly across the network rather than an attached drive.
So ... my advice before upgrading.
1 - Backup ... like sunscreen you will regret not having covered yourself.
2 - Don't just turn of Time Machine but unplug external drives where possible.
3 - Make sure you read up on the difference in navigation because when you are using the trackpad and suddenly go to LaunchPad, or MissionControl or swap between apps ... you know you are going to click on the wrong thing and lose a loooooong forum post you have been writing (and thank $deity for autosave!)