Good products: The Garmin GMA 350c audio panel. If you try it at Oshkosh or Sun 'n' Fun, the voice recognition sucks... But it was designed to be in an airplane, not in a tent full of people. When you use it in the airplane, it's awesome! It saves a LOT of taps on the GTN. Very glad we spent the extra $400 or so on it (compared to GMA 345).
Bad products: Anything by Honeywell/Bendix-King. It's clear to me that they don't care about the small GA market any more. Very poor (and expensive) support of their older products ($9600 for an overhaul exchange trim servo on my KFC150, for example, and they didn't even have any), and their new efforts are either outright bad (KSN 770 GPS, where you need to pull out the manual to go direct to something) or their certification efforts are so lackluster (KI-300) that they've gone three years since announcement and still aren't certified or shipping. Weak.
Good services: Poplar Grove Airmotive engine overhauls. Engines always come back from them running smooth, and they run them a couple hours in the test cell to do the initial break-in so you can't screw it up. We've gotten engines back from them that are basically broken in already before they're even hung on the plane. In terms of longevity, the last engine we owned the whole time went 951 hours between overhauls (Poplar Grove both before and after), and we recently sold one with over 3200 SMOH that was still going strong. Needless to say, they've earned our business for the foreseeable future.
Another: Skycom Avionics at KUES (Waukesha, WI). Have done some work with them on club planes (ADS-B upgrades, mostly), but lots with the Mooney. First of all, they have a fantastic component-level repair guy who saved us from having to do the aforementioned $9600 exchange on our trim servo, and also recently re-did our KX165 so it works like it's brand new. He's been doing component-level work on avionics for 30+ years. More recently, we did major panel upgrade: Removing GPS, Nav/Com, ADF, DME, audio panel, intercom, and a ton of associated stuff, moving another nav/com and the HSI slaving control, adding GTN 750, GTX 345, GMA 350c, and FlightStream 510. It was done *perfectly*. On time, on budget, and every single thing worked on the first try (with the exception of a minor issue that was fixed for free with a Garmin software update), even the stuff that wasn't so obvious: The GTN gets fuel flow from our old Shadin, and our (rare) dual music inputs both work perfectly... Every single thing that was left in the plane works seamlessly with all of the new gear. You won't find better work anywhere.