I've cleaned a ton of brass and many of my smaller gun parts, like trigger assemblies, bolts/carriers, slides, etc. with very good results. I'm just using the $70 Harbor Freight ultrasonic cleaner, and have been for a couple years. I'm pry going to get a larger more powerful unit in the near future, but the cheapy has been fine thus far.
For parts, the Harbor Freight model has a good "general purpose" frequency. You'll want to scrape off as much of the big goopy filth as you can, ultrasonics seem to have trouble getting globs of grease or cosmoline off. As stated, heat goes a long way in the cleaning process.
As far as solvents go, I'm lucky enough to shoot with a guy who has a P.h.D in chemical solvents and cleaning agents. I asked him what I should use for parts and believe it or not he said Dawn dishwashing detergent (about a Tablespoon or 2 for the model I have, pry 2 qts) and water. For brass, the same thing but add white vinegar. Anywhere from a few tablespoons to 20% depending how bad the brass is. I usually use a healthy pour for fairly dirty brass.
After cleaning parts I rinse them thoroughly with hot water, blow dry, and spray with a heavy coat of oil. Let the oil soak in for a bit then wipe as much off as I can. The ultrasonic will suck all the oil out of the parts so it's imperative to get oil back on them quickly after drying.