I'm wrestling with the QRP bug myself right now. On the pro side, an FT-817ND, internal batteries, and an LNR Trail-Friendly antenna would weight in at three pounds and would fit in my Camelbak MULE's top pouch. The down side of course is that it's another $700 I don't need to spend and it's only 5 watts. Compare this to the FT-857D, which I already have. Try as I might, I can't get the real-world pack weight down below eleven pounds: radio is 4.6lbs, LiFePO battery 3.4lbs, tuner & balun 3 lbs. Throw in some random wire, lunch, and a couple of water bottles and it mostly fills a medium ALICE.
Now, I could ditch the tuner and run 25w through an LNR TF antenna to get this down to eight pounds, but that's a serious compromise – 1/4 power and just 10, 20, & 40 meters, as opposed to full power on 80, 60, 40, 30, 20, 17, 15, 12, & 10 meters at a cost of hauling around three more pounds.
So... 3 lbs vs 11 (or 8, which is really a more fair comparison). Fits in the Camelbak and out the door... or I have to schlep around with ALICE. Another $700-ish to spend. Ugh, analysis paralysis. Maybe I'll take the middle route and get the LNR TF, run 25w without a tuner for a while, see how I like it.
Before jumping in to QRP, spend some time over at
http://www.voacap.com. Do a bunch of point-to-point predictions, and remember to set the noise level (lower right on the panel) to something realistic, like "Residential." Flip between SSB and CW, and power between 100 and 5w. Look at the coverage map results too. For my situation, it shows that QRP can be made to work, but QRO is a lot better.
Just... 3 lbs and fits in the Camelbak... damn, that's tempting.