Won't be putting hives on my plot, Rob.
I spoke to someone who has bees in his garden and he said he's never seen one of them forage on his flowers... they usually go a bit further afield, I'm told.
Thinking about it, I might see about putting some up above some land that's just used as a dumping ground for people who don't make their own compost (WHY???) as that way they should forage over all the plots.
Then again - I know for a fact some holders there use insecticide so maybe not.
For sure, top bar hives are less commercial than a National, having no super and all that, but I have a feeling that what's seen as normal commercial beekeeping is less than ideal. Each hive will almost certainly produce less honey than a national, but it may be better for the bees, and with the hives being so stupidly cheap (you can knock them together for pittance yourself) there's no reason not to have a pile of hives on the go at once, so the less-per-hive yield could well be balanced out.
One thing I found interesting was a sort of study of cell sizes. They took bees from a foundation system and put them into a top bar hive (I think) where they built comb slightly smaller than the cells found on the foundation. Then they took bees from that to a new hive where the cells got smaller still. Long story short it took about 7 generations for the cell size to stabilise and it was definitely smaller than you see in most foundation.
I seem to recall reading that they also built different sized cells in the same hive for brood and for storage. I may be mistaken there though.
I can't say anything definitive as I can't back a word of it up right now, but I do remember reading it with an open mind, no preconceptions (having never kept bees before) and thinking it seemed like there was something really quiete unnatural and probably stressful (for the bees) about conventional frame+foundation bee keeping.
I watched a video of some top-bar keepers who had no more protection than a smoker and a white shirt creating artificial swarms and talking about their views on top-bar keeping. If I can find it somewhere I'll give you the link. Don't hold your breath though - it was quite a while back now.
In the mean time...
http://www.lulu.com/content/815182
If you've got the space and already know how bees are, I can't see any reason not to have a top bar setup on the go.