I ordered one of the new 17" the moment the store came back up to replace a nearly 5-year-old first-generation 17" MBP (which I'd preordered after announcement).
1. My original 17" was my first laptop, and I got it because I was bedridden with an extended illness at the time--I wanted the biggest screen and most power possible and wasn't going ANYWHERE with it. I kind of got used to the big form factor, though, so I ended up doing a lot of my less demanding work from the couch instead of on the 27" iMac right beside.
I went with another 17" because I mostly use it as a workstation from semi-stationary locations, not as a portable; 15" is significantly more comfortable to tote around and use as a portable if that's your primary use of the machine. I also get a lot of use out of the extra screen real estate when working with a side-by-side web preview and code. The expresscard slot is nice--I'm getting an eSATA card for it, and may get a USB3 card at some point as well--although not the main selling point.
2. Web development (front-end CSS/HTML and some lighter backend duty), writing, light-duty Photoshop work (the more serious stuff happens on a 27" iMac on account of the IPS screen, though it no longer has nearly the speed advantage compared to my previous 17" MBP), batch image processing, on-site troubleshooting (network, etc), plus the standard web surfing/video watching/email/goofing around.
3. Couch, either with the computer on the table or on my lap with my feet up.
I'll note that while I actually prefer the matte screen, I ended up going with the glossy on account of the extra protection the glass affords. If the MBPs used IPS panels (so I could use it for color-critical work), I might have gone with the matte, though.