In August 2008 I wrote
this article.
I still have both the VP5700 and the ND2700, although I've upgraded drives a few times. In Nov 2008 I took them both to Laos for a fortnight's cycling (handling photos from my 40D and G9). Each card got backed up to both devices before formatting, and then each device was carried in a separate person's carry-on luggage.
Even if I had SSD drives in each, I would still want two separate copies.
I did find when reviewing my photos back home that I'd made a few mistakes in some scenarios (e.g. when shooting kids in a dark schoolroom @ f/1.8, focus/exposure/shutter are critical ). If I had reviewed the images during the trip I would have not repeated some of those mistakes, and if I had a laptop with Lightroom running on it this would have been straightforward and routine. However I didn't feel comfortable lugging my laptop around on a bike...
The VP5700 does have a screen to let you review photos (and watch movies, etc) but like all these devices it just displays the embedded preview JPEG, not showing you the actual RAW data. Especially on the 40D this made it hard to judge fine focus issues, and I ended up not spending a lot of time on this during the trip.
Mostly since then I've been using them as USB drives for external storage when on the road, but with the knowledge that if my laptop and/or card reader blew up I'd still be able to back up new photos. Also I tend to put a few movie files on the VP5700 at the start of the trip, as sometimes that's better than the offerings on long international flights. I do have a second battery for the VP5700.
I am considering selling the VP5700 now though (FWIW, I bought it through TechnoBox), so if you don't want the extra features like slightly larger screen, AV and voice recorder, or AV out cables, that might be of interest to you. As far as I can tell, the speed of operation is similar to the VP8870.
Note that with the current prices of hard drives and the prices shown @ TechnoBox, the most cost-effective way of buying these would be to get an empty device and then whack your own drive into it.