The power of the various plug computers available on the market generates a real interest. I'm not sure "what" I'd like to do, however the prospect of the possibilities available makes me want to do "something" with one.
It would definitely be easier to answer the rest of your questions if you did have an idea about what you wanted to get out of the device.
Either you have a specific purpose in mind, and you spend some time configuring your plug to that purpose, and maybe learning something along the way (but maybe that journey only takes you one or two days, then the job is done), or you want to experiment and play.
Both are valid reasons for wanting a plug, IMO.
With this in mind:
- Is there any distinction between the modules in terms of "easiest" to get to grips with? I'd ideally like to get a GuruPlug Plus (naturally!) however is this harder, in any way, to use than for example an original SheevaPlug? Likewise, all software that runs on one plug, runs on all others, right? E.g., Tonido software designed for the SheevaPlug will run on the GuruPlug?
The Tonido people would be missing an opportunity if they didn't make sure their software works on a Guruplug, but since no-one has a Guruplug yet, no-one can say for certain that it will work without change.
Systems that rely on wireless connectivity or bluetooth might be more difficult to work with, but we expect these devices to work out of the box. If you intend to switch to a different O/S, then the challenge of these devices might arise.
- With no prior knowledge of these devices (or any embedded systems apart from PIC microcontrollers) is it possible to self teach?
Do they come with sufficient documentation to really make interesting progress?
Probably not. The real knowledgebase is here, on this forum, on the wiki, and on blogs that others have posted on the wider 'net, detailing their experiences.
Unfortunately the plug computer "scene" has slowly become more and more quieter since my eye first caught these devices. This makes it appear a little daunting to a beginner... Is this the case? Is it possible to really start getting serious usage out of these plugs starting with no understanding? Likewise, my understanding on Linux does not really extend between some basic tweaking I did (generally following tutorials) on an EEE Pc.
Which begs the question of what purpose you would have in purchasing. There is already enough information here to help you make good use of a plug. I think the Sheevaplug fulfills both desires; that of being a useful and practical device, but also satisfying the intellectual curiosity with an educational, interesting (and achievable) learning curve.