I worked through Inviso onsite at Microsoft. Their agency staff were friendly, attentive, and always available to answer questions regarding employment status and benefits. As a contractor, the Inviso experience was positive. I cannot speak for the experience of working for them directly.
I enjoyed the company a lot, and the atmosphere was very positive. Management trusts the technical expertise and didn't micro manage, but was also there to help when needed. At the time of this writing, the company is in a precarious position with Microsoft being their only client, and with the direction Microsoft is headed, this makes employment there a little risky.
Worked here for 1 year, left when a contract ended. Great atmosphere, friendly coworkers, terrific benefits, wonderful company to work for.