I use the Opus and the MII10000 cause it works, is simple and kept me from having to use a cap, extra battery, ITPS (sp?) or some other means of getting the same functionality as the Opus. If cost is a factor, most will go with the other methods such as a inverter. It is really cost vs. simplicity.
The only thing that I would like to have with the Opus is the ability to have a built in choice of start up delay - I want to give the CPU and HDD a chance to cool down (A/C blows on them) before I put the power to them. My current HDD is getting corrupted due to heat + vibration.