We've been using a Current Cost Energy ENVI-CC128for a while now.
Good bits first. It is very easy to fit - the device works (I assume) by electrical induction. The sensor has a clip which fits round one of your mains wires, and then communicates wirelessly with the monitor. It sets itself up automatically, and it is apparently compatible with further units, which can be added and set up with similar ease.
If you are displaying this visibly somewhere, the feedback on power usage will almost certainly result in you increasingly switching off unnecessary lights and appliances. This is how it is likely to pay for itself. We have notices changes in our behaviour, and in the event that you don't know which the expensive lights in your house are, this will remind you.
Now the not-so-good. The sticky pad which is supposed to support the sensor isn't strong (or alternatively, large) enough; our transmitter thingy has ended up falling off where it was stuck. The velcro join is fine; it's the sticky pad that doesn't hold the weight of the transmitter when stuck to a smooth plastic surface. Maybe I needed to press it on harder, but I'm wary of pushing too hard on the side of the consumer unit!
Most of the displayed information is, to be honest, a little over-the-top and limited in value. The bar graphs have no scale, so don't really offer much except a vague comparison. The "cost" indication is all very well - it can be set to what you are paying - but the sort of person who can make sense of the concept of electricity units is likely to also be able to make deductions about cost from those values.
We are generating some of our own electricity (with solar panels). Unfortunately, the means of measuring energy use is indifferent as to whether net current is flowing into or out of the house. During the day, when we aren't using much electricity, and the solar panels are generating, we become a "supplier" to the grid. However, the device assumes that this is power being used, artificially inflating the figures for the day. It's fair to point out that this currently isn't likely to be a widespread issue ....
Finally, there's the issue of software. The data from the device is output through an Ethernet connection - but a weird Ethernet connection, which apparently has to connect to a USB dongle - which means that if you want to use this with a computer, you effectively need to buy another piece of proprietary hardware. Other people have commented on the quality of this - their comments, in my opinion, make me wonder whether simply buying a more computer-friendly monitor might make more sense. Also, too much of the software is only available remotely - your power usage being sent to a server on the internet, rather than being analyzable by you. I don't think this is the best way of doing it, and again, my hunch is that there are other devices that are likely to be more user-friendly.
However, if all you are looking for is a power monitor which will encourage you to switch things off, and show you what is going on electrically in the house, this is an excellent little device.