Pluspunten
OPI's greatest asset is its people. Many of the engineers were the best I've ever worked with. They are a great technical force in the Twin Cities and host lots of meetups, etc. OPI has gone through a challenging acquisition but, even though I left (more on that below), I think it would be a great place to work and I highly recommend it, as long as the core OPI leadership is intact.
Minpunten
OPI is awesome but your daily life will be almost entirely dictated by your client. This is normal for consulting, though. Their benefits were restructured post-acquisition. While Improving's benefits were still very good, suddenly with more standard benefits, PAY became more of an issue. OPI paid, imo, within the average for the Twin Cities, but maybe on the lower end of average. However... OPI engineers are very good. :) With the benefits suddenly not all that special, I left for a 55% pay increase (not an exaggeration). Even though I loved and still miss OPI, it was too much to ignore.