I'm using the Noland RS11 and I'm very satisfied with the results.
It's currently set up to monitor RPMs, oil pressure, coolant temp, and voltage from each engine. I could get more values if I bought a 2nd RS11, but haven't decided yet if I really need that.
The data displays nicely on my Garmin chartplotter. I especially like displaying the RPM data right on the chart, so I can always keep the engines in synch. As support for N2K grows, there will be more devices and interfaces that can display this data. Maybe one day it'll be available via WiFi to apps on phones or tablets.