I've created a nice app widget that I could not find on the market, and even took the time to publish it (more details at http://code.google.com/p/frequentcontacts/ ). I've been contacted by people having problems with this widget, and I don't know what to do about them. It work great on the emulator, and on my Nexus One, but one user reports about com.android.mms force close on HTC incredible, another about motor droid not displaying any data, and a third with nothing more than "doesn't work" without any further details. The widget does have more than 400 active users, so I guess it is not total crap, and I use it all the time. How do I help these users without buying more devices? I'm not even sure that is the real cause, but I can't be sure, since I've already seen differences between the emulator and my Nexus One.
Simulator is your best choice. You can hunt the forums for a way to install a custom ROM on the simulator to emulate exact behavior of different Android phones.
Look for instructions on http://forum.xda-developers.com
In addition (though I don't think it's the method for you, since this is a free project and you probably don't want to sink money into it) you can use a service like DeviceAnywhere, which allows you to test your software on actual real devices over the internet. You have to pay for it though, not sure about the pricing.
Some people will report that your app doesn't work, just because they don't understand something. These particular comments you're referring to seems exactly this case. Furthermore, fighting device compatibility for Android apps isn't worthed anymore - the fragmentation gets worse faster and faster, and you simply can't catch up. In a simple app you can be certain that you don't rely on some hardware-specific stuff and you should never bother figuring out what's happening on somebody's Sony-Ericsson, LG, HTC, whatever. Once you start putting
if (Build.MODEL.equals("Nexus")) {
//...
you've got yourself in the fragmentation hell.
See Be careful with content providers. I once found Htc device returns different data from emulator using the same URI. That's why fragmentation draws much attention recently. Painful.