Me and my team use Paymo.biz for almost two years now and we're pretty satisfied with them. Why we use their app? Well to be honest the good price is something we considered + it's pretty easy to use, it has a simple structure.
The design is a bit outdated, but they're working on some new version, so that's a plus I guess.
I use http://www.paymo.biz and although they're mostly known for pm & time tracking they have some nice invoicing options: estimates, expenses, invoices & reports.
I'm suggesting it because although I use a paid version, there's a free one too, as I freelancer you should keep your expenses down at first am I right? :)
I solved that by using Paymo Plus (a tool from Paymo). Although I set up projects and everything in the online account, I use Paymo Plus on my PC to automatically record everything I do in a day. It records everything, browser, PDF editor, text editor, IM, etc. At the end of the day I just link the time entries and voila, I'm ready to bill the client :)
I use it because I'm not a big fan of starting and stopping for each task.
I think it depends on the sites in questions. If they're unethically filling every pixel of screen size with adds, overlapping adds etc, then I think some addblock is just what the doctor prescribed.
Site owners need to have some common sense when it comes to advertising
I suggest to use Workflowy and Paymo Plus (automatic tracking tool), these two together will help you be organized and know exactly how much time you put into the project.
I don't know how complex you want it to be, but the tool I've been using for almost 10 months now is pretty great. I think you should take a look at Paymo ( http://paymo.biz ) maybe it's up your alley.
as I see it, you must think of cost, how much of a bargain would it be to have me the potential buyer purchase such a device, and not have two separate ones instead. Another key aspect is the battery life, big screens mean your battery won't last that long, and it would suck to have both your phone and tablet dead.
I think if these 2 are resolved, people will consider buying such a device.