I've been adventure racing for a number of years now and mountain bike orienteering since before that, and always looking for the perfect map board for the bike. I think the Intrepid AR51 Map board from Area51 is as good as it gets, and here's why.
Simple attachment: Only two bolts are required to attach the board to the bike and hold it securely which saves time attaching and removing the board. In adventure racing we often have to pack and unpack our bikes at the end of each bike stage. With the Intrepid AR51 map board, all I actually have to do is loosen one bolt and I can swivel the board so that it fits into my bike box.
Velcro closure: I've used boards with button closures which is fine if you just have a single A4 sheet. In a typical adventure race though you often end up with a map folded multiple times or even several maps on your board. The velcro enables more leeway with the thickness of a map or maps that you can fit on the board. It is also much easier to secure on the go. Stopping to change or fold maps takes time, so often I do this while riding – the velcro make this much easier to do. (Be careful when doing this though!)
The right size: Basically as big as you can without getting in the way. The Intrepid AR51 map board seems to be about right. It's much better than some smaller boards I've had. Bigger means not having to fold the map so much which saves time, but not so big that the size becomes a problem. I can mount my bike computer beside my handlebar grip and still see it with the map board on. The bike computer is an essential tool for navigating where the map doesn't perfectly match the ground (i.e. always).
Stiff top board: A map board is no good if the top board is so flimsy that it shakes around too much for you to read anything. The Intrepid AR51 top board is great for keeping the map stable while on the move. As I mount a compass on the board as well the board needs to be able to handle the extra weight of the compass, the Intrepid AR51 does this nicely.
As an aside, here is the reason why I prefer a compass on the map board as opposed to anywhere else. I have a good quality orienteering compass with a rotatable bezel. When I load a map onto the board I can then rotate the compass bezel so that it lines up with magnetic north on the map. From then on, to orientate the map, I need only look at the compass and swivel the board so that the needle lines up with north on the compass bezel. I don't have to look at the map and work out where magnetic north is again. It's accurate and saves mistakes.
Finally, once all the above criteria have been met, you don't want to be lugging around more weight than you need, so it's good to see that the Intrepid AR51 map board is still lightweight and even due to receive a bit more trimming in the weight department for the next iteration.
In summary, the Intrepid AR51 map board does everything a map board for adventure racing should, at a great price. You can't go wrong!