I'm using the HMC631 evaluation board and having some strange issues:
1) The minimum gain point should nominally be at I=1.5v and Q=1.5v (perhaps +/-100mV), I followed the application note to try to find the true Vmi/Vmq values, but it didn't look like a very strong minima. Instead I used a programmable PSU to sweep the I and Q voltages - it turns out that the minimum gain occurs for me at I=1.6v and Q=2.9v. (you'll note the voltages are out of range of the 0.5->2.5v range, in fact the only way that I've been able to realize the datasheets quoted 40 dB of gain range is my using this larger voltage range).
2) My power supply is not actually able to reduce the I/Q voltages down to 0.5v, 1.1v seems to be the max. No idea why this is - probably related to the first problem?