We have used Impero for the last couple of years and are delighted with it.
As previously posted it does sometimes go through 'development phases' but these are always short, infact the developers are top rate. We have had positive dialogue with them several times in terms of new feature requests as well as bug fixes.
One of its major strengths is the fact that it offers much more than punative / restrictive functionality - as well as the support tools, like mimicing one PC it also allows for things like quick 'straw polls' in the classroom, or having one student broadcast (EG demonstrate) as skill or a solution to all other workstations in a lab.
Hope I'm not gibbering as I'm a little rushed due to running off for a meeting . .