I'm an Emergency Medicine doc and have responded to several in-flight situations.

I have almost universally found the flight crews to be great in this situation.

There is limited equipment on the aircraft: oxygen, a first aid kit, and automatic external defibrillator. There is also an advanced medical bag on the aircraft that contains IV's, fluid, a glucometer and some medications used for cardiac arrest. You have to be a licensed healthcare professional to use it and you have to ask for it. I've personally started a few IV's in-flight and have administered glucose to patients with diabetic emergencies.

In every case I've responded to the flight crew deferred to my decision about whether or not to put the plane down. I've only requested it once the the flight deck crew did not bat an eyelash. Plane was landed at the closest appropriate airport.

As an aside, for the last two emergencies I got a very nice thank you letter and gift basket from DL. Not necessary, but nice none the less.