I've been using my SP3 for green laning recently. It's not the perfect tool for the job, but for the price it's very good. As Burnie points out, if you went for the more suitable 276C it would cost a lot more, as you have to buy the maps seperately.
I was thinking of upgrading mine to a Quest, but I was put off by the smaller screen (motomartin's wrong about the screen, the Quest's is much smaller than the SP3's) and the seemingly flimsy mounting solutions, although Mr IFan tells me he's had no problems with the Quest mount he's using for off roading.
As for the lack up support for OS grid coordinates and the correct datum, I haven't found it to be a problem really. If you want to know where you are on an OS map, you can quite easily check the GPS for nearby towns and cross reference to the paper map. Trackback is probably a useful feature but I haven't ever thought "I wish I had that on my GPS" but possibly you don't know you need it until you've got it, if you see what I mean
In summary - If you want cheap and cheerful, and don't mind slow routing calculations and map drawing, go for the SP3. The Quest is faster, smaller and about twice the price. The 276C is even more expensive, but fom what people say appears to be the ideal green laning choice.