I just got my MBP15 today - much to my dismay, it did not come with ML on it already. Not a huge deal, since I was planning to replace the HD anyway. In fact, before I even turned it on, I replaced the HD with an SSD and replaced the 8GB of RAM with 16GB of RAM.
I downloaded ML and made a USB flash drive boot disk - that process was painless, once I had the download (which took 3+ hours). While I was waiting for that to finish, I did a clean install of Lion on my SSD drive, by cloning the new HD that came in the MBP (now in an external enclosure for now) to the SSD. That let me boot up the new machine from the SSD. Once I had ML installed on the USB flash drive, I booted from that, and it updated the version on the SSD. Long story short, clean install, but mostly because I always start fresh (I'll do a clean install on my machine every year). After the ML install finished and I rebooted, I ran software update (which now runs through the appstore, which sucks), and it wants to update iPhoto and iMovie already. So, that's what it is doing. I'm about 25% of the way through the iPhoto download. Overall, all these downloads take forever.
As it stands right now, I have a clean install of ML on the SSD, currently being made up to date, a clean install of Lion on the original HD - which I will wipe out once I am done fooling around, and a ML bootable installer on a 32GB USB2 flash drive.
Next, I will replace the optical drive with the original 750GB 7200 drive that came in it, using the OWC Data Doubler I've got, and put the optical drive in an external enclosure, also bought from OWC.
Then I have the unenviable task of making myself at home on the new system - installing applications, reconfiguring my environment and tweaking things, setting up accounts, and finally, bringing over my user docs and stuff. This crap takes forever to do - but this should be a big upgrade from my previous machine.
Note: I had to partition my 32GB USB flash drive. Otherwise, the ML installer process somehow takes up the whole drive with a single 4.XGB volume, causing an effective loss of the other 27.XGB.