A couple of times, but not a fan of cycling with the laptop in the panniers (not so fussed about the £40 tablet!). So, HP Windows laptop running PZTLog and my homebrew "passive" data interface: No PTT, just isolation transformers in a box with a 4-pole 3.5mm (combo socket on laptop) at one end and a 6pin mini-DIN for the FT-857.
(scroll to bottom of this page) http://www.m0pzt.com/datamodes-interface-for-jt65-psk-and-rtty/
Main issue is dealing with laptop screen and sunlight - for a lightweight /P like me (cycling to all my /P jollies) you do need the comfort of a tree, parasol or hatchback+blanket.
There is actually a £10 kit over at Ham Goodies which contains all the bits for a "standard" interface: Transformers, Opto-Couplers, USB/Serial converter etc to handle both digimodes and CW keying. The user just adds a box and whatever cabling they need for the radio - Homebrew at its best, I say. I've also got those 4-pole 3.5mm combo/tablet connectors in "easy-solder" form - unlike the type Maplin sell The kit is aimed at "Intermediate" level but the supplied instructions give you all the details to complete it.
...and now for some balance: Those £25 ones from RadioArena aren't bad, either - although I did notice that the (stereo) input only came through on the right channel so had to modify PZTLog's datamode engine. They are available in several formats depending upon your radio. If you really didn't want to make one, then this is a fairly good solution.
I've operated digital (PSK31) while portable. I use an Android tablet and a Wolphi interface to my FT-817nd along with an LNR 40/20/10 end fed antenna. It makes for a small and light package. For longer sessions I also have a 12ah gel cell but that alone weighs more than the radio, table, interface and antenna combined!
I think i'm going to look out for a cheap Windows based tablet with a TRRS jack.
I'm going to make a bluetooth CAT dongle for the 857 so I should be able to get full control via that and just need the TRRS jack for audio in and out. Seems to be quite hard to find a tablet that actually specifies a TRRS jack though
The Linx 7, 8 or 10 (in those screen sizes) are probably worth a look - been around for a while, and there's a look at the 8" version over at Essex Ham: http://www.essexham.co.uk/news/linx-windows-tablet.html I've had a go on it, and it's quite nice. Having seen the 7+8 together you would almost certainly want the 8 for the slightly larger screen. Just add one of those cheap USB keyboard leather holders (about £3 on Amazon). The Linx you can get in Staples. The 10" is interesting as it can be used with an "origami" keyboard (it folds open and props-up the screen like a photo frame). That has a special docking connector so you don't lose the single USB socket. There are some issues with OTG adapters and charging but there's a forum with tips/tricks and links to cheap and known "good" OTG splitters etc - assuming that you may need to charge it off your SLAB when /P. If not, I'd say about 4hrs operating time.