There are a few of us on here with Active 10s. It depends on what you want from a GPS whether it works for you or not.
I don't particularly want a GPS that tells me exactly when to turn, so the somewhat simpler overlayed route onto the OS mapping suits me fine. I can use it to follow a route, but I can ignore it and divert off and rejoin as and when.
The OS mapping is very good, especially if you want to know what that thing on the other side of the fence is that you've just cycled past (assuming that the OS indicates this on their maps).
The downsides for me are:
(i) the mapping can be a bit expensive, £200 or so for the entire UK at 1:50000. If you wanted the same area at 1:25000 it would cost you something like £4000 (iirc) ! I've got the London AtoZ at 1:14000 and the bottom half of the UK at 1:50000 (around £100) which suits me fine at the moment.
(ii) The screen can be a bit difficult to see in bright sunlight. It works great at night, and you can turn the backlight almost off, which improves the battery life. In bright sunshine it can be a bit of a bugger to see from some angles. Luckily bright sunshine isn't that common in the UK (!), and it's generally fine in dull conditions.
(iii) The design isn't as robust as I'd like. You can get water behind the screen cover bit, although that can be removed and dried off. On mine the cover over the sockets, and the rubber bit over the joystick have both failed. I've had to send it off to Satmap for their General Service, which has cost me £35 (£25 for the service and £5 each way Special Delivery).
(iv) You can only see a limited amount of each map at a time, obviously due to it's portable nature you only have a small screen, and there's not much you can do about this. I find that I either need a paper map, or something like Bikely to do my large scale planning on, but I suspect this would be a failing of any device like this.
Having said all that, I love it. As far as I know, it's the only way you can carry this much mapping around with you, and the OS mapping is of excellent quality, so it's great to have the equivalent of 100 or so Landranger maps in a device that fits in your hand.
The Li-Ion battery pack(s) are a good idea. You don't need to buy the power bundle, just buy a Li-Ion battery, and charge it using a USB port on your PC/laptop and save £20.