This is a good question that I'd love to hear others input on as well! I'd also love to hear more about your system! I think your on the right track, one the most widely promoted way to use small-scale biogas is through a cookstove. It is the main thing we use our biogas for here at ECHO. It's also probably the easiest use. A simple conversion of your propane stove (enlarging or removing the jet orifice) will allow you to use it right away. Another relatively simple use would be to supply gas lamps (we are currently looking to do this at ECHO, but haven't installed any yet). Fueling a boiler or something like that would be fairly simple to convert as well. When you get into internal combustion engines, it becomes a bit more complex. While biogas is an eco-friendly fuel even in an IC engine (it is carbon neutral because the feedstock comes from today's plant, animal, and human waste, things that would otherwise be part of the natural carbon cycle), it becomes a bit more complicated.
This complication is due to the need to purify the gas. Biogas is typically composed of only 60-70% methane, the rest is mainly carbon dioxide (CO2), nitrogen (N), water vapor, and hydrogen sulfide (H2S). While it accounts for a very small percentage H2S can cause some of the greatest harm to your internal combustion engine. H2S is very corrosive. Throw in a little water vapor and you have a recipe for rapidly deteriorating your engine. Appropriate technologies can be used to remove CO2, H2S, and water vapor, but it all depends on how much work you want to go through to maintain the system. The costs may outweigh the benefits, but that all depends on your specific situation.
I hope this helps a little! Please keep us updated on how you decide to use the gas!