I had a 13" early 2006 MB, replaced it with a 15" early 2008 MBP.
I have this one hooked up to either a 27" 2560x1440 Dell Display or 3x 1280x1024. Depending on where I work/stay.
Now I'm really used to the screen real estate so there is NO way of NOT getting a 17" MBP. As for comparison. While the 17" MBP might seem huge, stack it on top of a 15" MBP and you'll notice that it is only about an inch wider and something around 3/4 inch deeper. The one pound difference is also not THAT much of a difference if you have the right bag.
Also, while 15" might be the 'gold standard' right now. Take a look at most PC notebooks. 15" wintel machines are HUGE. If ppl can use and carry THESE, why would you have a problem with a 17" MBP?
Or... for another comparison look at some bag mfr. Booq for example. Their bags state something like 'fit 14"-15" PCs or 15"-17" MBPs'.
Also just for comparison. I saw this here a couple days ago. I knew it existed... but never bothered checking its dimensions.
So take a look yourself!
http://www.dell.com/us/p/alienware-m17x/pd
quote from said page:
Dimensions
Height: 51.31mm to 53.59mm (2.02 inches to 2.11 inches) – Front to Back
Width: 405.89mm (15.98 inches)
Depth: 321.31mm (12.65 inches)
Weight: Starting at 5.3kg* (11.68 pounds)
:unquote
This one (maybe a bad example as it is meant to be a mobile gaming powerhouse... but still) is like TWO 17" MBPs stacked on top of each other!
The 17" is definitely worth it, regarding the screen and ExpressCardSlot alone.
The only problem I could see/envision is on public transportation. As, except for some trains, there is limited space.
My sidekicks are an iPhone 4 and an iPad. I pretty much never ever go out without them. Do I get work done with them? Nope. Except for emailing and some notes. But I'd hardly call that work.
If you want my 2ct... If you got a 17" MBP... you might like to have an iPad. But personally I think any MBA would be rather useless. But then again, I just HATE having to sync files b/w different computers, and the use of my iPad during travel doesn't warrant a 'real' computer.
But then again YMMV.