If you're going to use accesskeys at all then yes - stick with the UK Govt recommendations. Accesskeys are notorious for overriding existing browser shortcut keys and you'll cause more annoyance than anything else if you do that. At least the UK Govt recommendations don't contradict anything (unless afaicr you're using Firefox on Linux or trying to enter a special character using the Alt+0123 number sequence).
The jury's still out on whether accesskeys are even needed though.
Regarding your second point, I'm really not sure. I went through a phase of using them just because I learnt about them, but I can't say I've used them since. They could be useful for keyboard dependant users I suppose...
Suddenly remembered where I'd seen you around before Sandpetra - Accessify :)
Accesskeys are a bit like the bad old days of DOS applications -- no standard key shortcuts at all: you ran the risk of hitting F1 in one application for help, and having it exit without saving, or starting to print.
If you do offer accesskeys, best to not make for even more non-standard combinations, so uk UK.gov recommendations are a good start:
How do visitors know you are using them? You need to tell them. As the UK.gov page says: When this navigational system is made available, it is important to inform your website users, as soon as they enter.
It is good to offer accesskeys -- they are the only or best way around sites for some visitors.
When deploying accesskeys, how do people (who would benefit from using them) find out what they are?
The suggestion is to create a page that contains what is referred to as the Access Key Bindings. This shows/tells the visitors what Access Keys are available and what keys are assigned.
I've been using them for over four years now and haven't had any comments positive or negative about them. I like them because I'm a keyboard power user so I can easily navigate specific areas of a site without having to touch the mouse. Particularly useful on a laptop with a touch pad. I still, to this day, cannot work with a touch pad, I just can't do it.
Access Keys are not universally supported. The following is a brief outline of who supports what and how they are invoked.
Internet Explorer focuses on a link when an Access Key is used but it does not follow the link (you must press your Enter Key to invoke the link). It will also submit a form if an Access Key for the submit button is used.
Mozilla (Firefox) follows a link when an Access Key is used.
The Access Key is one of those things that adds a level of accessibility for certain environments. The UK guidelines were written for UK Government websites and do not necessarily apply to all websites.
For example, let's say you have a busy message board or forum. You might take the 0-9 numbers and assign those to various Access Keys for your users. As long as you have a quick link to a map that contains the Access Key Bindings (what keys are assigned to what number), you can do as you wish.
Just keep in mind, that there is a chance that your Access Key Bindings are going to conflict with preassigned Bindings from other programs. Using 0-9 minimizes those conflicts but they still exist. Using letters really opens up a can of worms and you should be aware of what may or may not happen when using alpha and numeric Access Keys.