I carried an iPad for the entire Northern Tier last summer, and I got all the power I needed for it, my camera batteries and my cell phone from my front hub generator and an E-Werk. I imported my photos and wrote a blog entry every night in my tent and uploaded it whenever I could find free WiFi. Only in Glacier NP was I unable to find free WiFi.
I didn't use a GPS, so I can't comment on that, but I very much liked the iPad for daily blogging and annotating photos. This was particularly valuable for me because I'm a college professor, and I teach a course entitled "Geology of the National Parks" each semester. I visited Olympic, North Cascades, Glacier and Acadia National Parks along the way (plus many other examples of interesting geology in between) and did a LOT of photography for classroom use.
It was great to be able to do some photo editing along the way and even create annotated presentation files (using Keynote) that I'm using in my class this semester. I was self conscious about the daily blogging, but I was pleasantly surprised by how much support I received from my university. I suppose it helped to demonstrate that at least some of my two-month vacation was of value to my teaching
I would not want to have done this with a smart phone / camera combo. Although I didn't carry an SLR-type camera, I used a high-end compact, which was much better for my purposes than any phone-based camera. It was great to be able to pull the photos from the camera each day.
Anyway, my point is just that whether or not carrying a computer or tablet is worthwhile for you depends on what you want to do with it. It was definitely worth it to me, but it wouldn't be worth it for many.
I found carrying the iPad to be quite trivial: I got a cheap padded zippered case for it, and it nicely slipped into an integrated sleeve in one of my front panniers. Although the typing experience isn't ideal, the lack of keys or a hinge makes it very easy to care for. For me, the iPad was a great compromise between weight, power management, ease of protecting and usefulness.
If interested, here's a link to the blog:
http://hpscott.wordpress.com/