Hi All
I installed an Oxyvent tank two years ago into a new build (Detached house in country). It is a condenser oil boiler heating system with rads. My plumber did not charge me any extra to install it. As far as I am aware it is relatively easy to install. As for how it has performed, I haven't had to bleed any raidators and my heating bills appear cheaper than similar sized houses. But I guess its all relative regards how often heating is on etc. I have TRV's on all rads and 4 zones in the house (Upstairs, downstairs bedrooms, downstairs living and hot water).
However I believe the tank does work, have no cold spots on rads and would believe it to repay for itself in a relatively short period of time. When you think of the amount of money you'll spend on other "necessities" in a new house and with the rising utlities prices I think its worth spending the money on the tank.
As for opinions of plumbers or engineers. My plumber thought it was gimicky at first but could understand the principles once he installed it. I think if you look at any new technolgies like this when they first come out (underfloor heating, wood chip boilers etc) you'll not get an over enthusiastic reception from guys who have been installing oil burners for thirty years and then need to change their work practice to what may be a new fad.
Anyway thats my humble opinion on the Oxyvent system