I've responded maybe 8 or so requests for assistance -- all on international flights on a mix of airlines. I'm well-practiced in pre/out of hospital/resource-constrained care, so YMMV, but here are some personal thoughts:
1) Many here have expressed concerns regarding scope of practice, specialty, etc. I kind of say "to hell with it". As one or two posters have mentioned, you don't know what's wrong with patient until you go see the patient. And if you find that it is thoroughly outside of your specialty, you can certainly defer to medlink or another medical professional onboard. On one occasion a pax passed out in the aisle in E- on a 777 approximately 15 minutes before we started the Atlantic crossing. The other responding physician - an endocrinologist - who had arrived first deferred to me but remained available essentially as a consult or to discuss things. To me what it comes down to is I'd really rather have some kind of medically licensed/certified person assessing/overseeing the care than an FA with Medlink support. And on that point -- introductions to other people assisting are important. "Hi I'm aero219 I am a [MD/DO/PA/NP/RN/EMT/CPR] and practice [specialty]".
2) Both UA and LH are extremely well-stocked (on longhaul widebody metal, at least). As others have noted, quantities are extremely limited. You will not be running a 30+ minute code (you'll find the emergency will end quite quickly) - and then again, that's not really the point of the medical kit onboard. After a recent incident on LH, I was speaking with the Purser and was proactively offered the inventory list of the kit they carry, including medications and quantities. Happy to share with anyone who may find it useful.
3) Crews are generally extremely grateful and willing to help, but will require clear direction (think intern on their first night alone). I've had crews that have without instruction held blankets up for privacy, etc. before we could move the patient to a galley - and many have basic first aid training, but have never used it under stress (i.e I asked for a glucometer and was handed a defibrillator). I've found that the offering of compensation is usually out of the crew's gratitude for not having to deal with the situation on their own and wanting to do something nice for you without awareness that it may impact liability. On the other hand, the law does note "intent" and I think it would be extremely difficult for any attorney to prove you only assisted for whatever potential "compensation" might or might not be given that varies by airline, day of week, position of the sun, etc. Also worth noting that when assisting on a non-US carrier, laws of the Flag of the Ship may apply.
4) After only one of the flights did I receive a very nice follow up email from corporate. But then again, there were two medical emergencies on the flight, maybe about 15 minutes apart and I got really worried I was about to unwillingly enter "fish for dinner" territory.

tldr:
If qualified: provide help
If treated; Given the circumstances, don't sue, thanks!