Hi
Yes the watts up meters are great, I have 3 of them! and use them to monitor charging on packs and one on the bike.
It goes up to 60V and will retain the measured values as long as the main power is connected or you hook an external battery.
On my bike I have my watts up mounted on the top tube on one of those brackets that you use to hold a bike lock on the frame, its handy as i can keep a little 9V battery inside that and connect it when I really want to keep the measured values when I unhook the main power to the meter.
They are great value when laid next to a drain brain as they do almost exactly all you need, I have a cycle computer so I don't need that function, all I am interested in is the Min low volts, the Max amps drawn and the watt hours used that's all you really need to know? the meter does more however.
I know the new drain brain is back lit and waterproof which is a nice feature however on my bike I don't ride and look at the meter at the same time as this is very dangerous esp. at night and I cover it with a little clear plastic bag for rain protection.
The drain brain is a great piece of kit but it was a little too pricey for me the watts up meters are cheap, easily available and come in nice snazzy colours as well!!
There are other options as well, I like the Medusa meters as they log as well and you can download it to your PC via USB.
http://www.medusaproducts.com/Power-Analyzers/PA-60100T.htm
http://www.powerwerx.com (look for power meters)
Cheers
Knoxie