Having just returned to Oz from my epic 6 month travels, I have a couple of different experiences to share on this topic.
First flight: Turkish Airlines J from IST-GRU (14 hours, much improved by the J flatbed!) - about 3 hours into the flight the call comes out for a doctor. Pt is (of all things) Bulgarian, and poor English, but has had allergies requiring hospitalisation and adrenaline before. Of course, he hasn't bothered to be tested for an allergy trigger or get the adrenaline pen he's been told to, and now his lips are starting swell....

Long story short, I get him to a quiet spot, track down the adrenaline (in case of disaster) and then a Brazilian doctor shows up who thankfully has some oral antihistamines, which work. (Darn, no chance to use a bread knife and a biro to make an airway!). Of course, then they work out where I am sitting... so I get called to someone with a headache ... yes, seriously (Medical advice: Panadol and water for dehydration), and later someone who feels faint. However, by the time I make it from 4D to the back of the plane, the hard-working Brazilian doctor is already there (she'd also been checking on allergy-guy through the flight) and shoos me away, pointing out the obvious fact that both she and the patient speak Brazilian Portuguese and I don't. Although the crew are grateful to us, there is no effort to take our details, there is not even any forms to fill in (I made my own casenotes on my laptop just in case) and when I ask about Medilink or any other medical advice services the crew looked at me blankly.
When I got back to J, I asked the crew if there were any spare J amenity packs and they pull one out and wave it at me, looking rather confused when I ask them to take it back to the poor Brazilian doc in Y. When I pointed out that she was stuck in cattle class and had been doing free work for them most of the flight they were a bit surprised and said "Oh, that's really nice of you to think of her...."
In stark contrast was my trip home the other day - QF Y (exit row without fee, not sure how I managed that!) HKG-PER. About 3.5 hours into the flight, a doctor page comes over the PA by the CSM (Wendy - a great CSM, and the rest of the crew were wonderful too). Since she's already brought my neighbour and I J blankets for the chilly exit row, and I am awake and sober, I respond. Minor case - vomiter who fainted for a second. By the time I see the passenger, they've finished vomiting and are feeling a bit better. I gave them some of my own stash of anti-nauseants (Ondansetron, for the medics reading) and suggest fluids and lying down. (Yep, not genius advice I am sorry to say!). CSM pops the passenger into J (Skybed 2) to lie down, then finds me for some paperwork and thanks me profusely. Half an hour later, CSM reappears with the dutyfree catalogue and tells me to pick out something as a token of appreciation from Qantas... Unexpected and probably undeserved!
Unfortunately before I get to finalise my selection, Wendy reappears and tells me there is a passenger with chest pain... I start to feel less guilty about QF's gift, realising that I might be earning it shortly, since we are still somewhere between Bali and NW WA. I manage her to the best of my ability (reasonably stable) and then wait a half hour or so while they try to get permission for me to come into the coughpit. After the powers-that-be decide that I am not planning my own 9/11, I get to go into the coughpit, which was pretty exciting! (Yep, adult, 55 flights last year and still excited by that... sad!) I get to speak to Medilink in the US via the coughpit radio - thankfully they agreed with what I'd done and didn't recommend any diversion (as wonderful as Port Hedland is...)
For the record, I got a lovely watch - and also a travel adapter for the poor long-suffering woman sitting next to me who got no sleep overnight! Thanks to QF, Wendy, the pilots/officers and crew who were wonderful and helpful people and I have no doubt they would have managed things brilliantly without a doctor onboard.