A lot of home studio folks suffer from FoO syndrome (Fear of Omnis). This is related to FoB (Fear of Bleed) and FoCRS (Fear of Crappy Room Sound).
In most cases, these fears are entirely avoidable. What is missed here is that because omnis don't have proximity effect, one can usually put them closer to the source than is practical with cardioids. So even though they are theoretically picking up room sound or bleed form other instruments, because they are so much closer to the intended source than to the room walls (or to neighboring instruments), the signal to "noise" ratio is quite high. In addition, whatever distant stuff gets into the mic will sound much better through an omni than a cardioid, because the off-axis sound is far less colored when using an omni.
So, I definitely recommend trying omnis without fear on instrument sources! I guess I'm agreeing 100% with what Gobblefisher wrote earlier. Must be something about being around Boston...