Hi. I'm really interested in jhana too. As a fairly new practitioner in this community who has worked with both vipassana and jhana, I have some thoughts on your post. I'm no jhana master and I don't claim any degree of enlightenment, but I do feel my "toes are wet", so to speak.
The first thing that comes to mind is public observability. There's a general philosophy of science thing that phenomena should be publicly investigable by any interested party. So there are some well known problems here with disagreements on the exact definition of jhana, and just the uncertainty around the experience. This is evidenced by the continuous flow of DhO discussions on "Is this first jhana", "what about this"? etc, etc. If DhO is any measure, jhana diagnostics don't seem to be evolving in a useful way even after several years of public discussion.
I've seen a lot of different definitions of jhana, and I've also had varying experiences with jhana. You have the "strict hard jhana" camp, who will claim that it isn't jhana unless it is so hard that you lose contact with the external world completely, that all agency is gone, and that you basically set yourself up, go down the jhanas as far as they happen naturally, and then naturally come out. Of course, there is some variation even in this camp, but I've heard people who I consider to be in this camp explain things roughly like that. I personally find more reason in the camp that distinguishes between hard and soft jhana, and varying degrees in-between. That has been my experience as well. In sitting meditation, I've hit jhanic states that were fairly "hard", with perception of the body mostly disappearing, thoughts basically disappearing completely, or at least, not being perceivable, and jhanic factors extremely obvious. But I've also experienced going through jhanas (and this is usually the case, as I wouldn't consider myself to be a strong jhana practitioner) where the external world is still perceptible, thoughts are still fairly noticeable, and where jhanic factors are faint. I still consider this to be jhana though. It's like a radio. Hard jhana could be considered to be where you're in a bath, all the lights are off, the atmosphere is silent, and the radio (the jhana) is extremely tuned in and clear, with a high volume. You're clearly listening to the radio. But then there's the case where you're walking the dog, you have a crappy portable radio, there is some fuzz, and you're still experiencing things other than the radio, but you're still clearly hearing the same song, with the same lyrics. I personally think that it's goofy to say that in the first scenario, you're hearing the song, and in the second, you're not. Varying degrees, IMO.
However, as far as pragmatism goes, even if we're still not at 100% consensus on what exactly constitutes jhana, there is definitely value in the jhana diagnostics that are going down here. Consider the first camp as given in the examples above. If you've never heard the radio before, and one camp says that the first scenario (hard jhana) is the only true experience of hearing the song, you might be walking the dog, still perceptive of the outside world, but if the definitions you've heard are so rigid that you don't bother to listen to the song because your focus isn't 100% tuned into the radio in seclusion, you might get frustrated and consider that you'll never hear the song till ideal conditions are met. This was the case with me, as I tried to practice based on more traditional, "hard" jhana definitions. I basically had no success. But after reading Ingram's book and reading around the forum a bit, I almost instantly became able to hit very soft jhanas. Then I got better and the jhanic experiences started getting deeper, and it was clear that they were indeed jhanas. So there is just one testimony of how the discussion and experience-pool here of how jhana can manifest can be of practical benefit.
As far as insight goes, I've usually been inclined to practice a more dry-insight technique (more bare noting), as it doesn't require any state to practice from. But I've also noticed that in times when I do jhana and then switch to doing vipassana, vibrations are much clearer, and equanimity is much higher. The mind is sharper, and nanas are more distinct and noticeable. Rising up through the nanas is easier and each bit seems more distinct.
In summary - Daniel's done a lot for jhana-awareness - gratitude and respect! But for me, operationalized definitions of jhana are still lacking: We don't really know how to get it, who can get it, how to really know you've got it, what can help you get it if you're stuck, etc.
Setting aside the notion of incredible variability from practitioner to practitioner, tradition to tradition, and tradition to tradition, my impression is that a lot of people are able to get jhana much easier after generally getting the sense that it can be done right now, that it doesn't have to be "hard", and having people describe the states in Y2K terms, rather than 2000 B.C. terms.
How to know you've gotten it? In my opinion, if you hit a jhana hard enough, the factors are fairly distinct, such as the bodily bliss, mental joy, diffuse happiness, silent, panoramic awareness, sense of expanding into space... etc... With all the variability, these factors seem to show up in people's descriptions, and I personally see a lot of consensus on this, even through debate about what traditional jhana is. Besides, does it really matter if you know you've gotten it or not? If you could invent your own jhanas, or something like jhanas, or something not like jhanas, investigate it thoroughly and see the 3 characteristics, and gain some sort of insight that benefits you or at least points you to a place of more informed practice/living, who cares what it was? As Daniel says on the front page here, pragmatism means what works is key. If it doesn't work, read more, put it to practice, share experience, talk to others who are working towards the same goal, or have accomplished the goal to whatever extent. I love to have things neatly laid out and well-defined, but at the same time, without actually putting things into practice, who knows whether or not any of these states/stages/definitions/maps/models/testimonies have any credibility or use?
In summary:
The first thing that comes to mind is public observability.
Not really important. The information is there, and there are people here who will be willing to debate and discuss definitions based on intellectual knowledge as well as direct experience, but what you can take away and put into your own practice is all that really matters.
The second thing that concerns me is the issue of jhana-talent.
Just my personal opinion, but I think that basically anyone with patience, a general idea of what they're going for, and belief that it is possible, will be able to develop jhana.
The third thing is the issue of substances as jhana-enhancers.
I've had good luck without any coffee, but I've also had pretty good luck with coffee too. Caffeine can make me alert and interested, which does help. After awhile though, once you get into jhana enough, you should be able to get into jhana just by remembering the states and getting a feel for how concentration expands and how the factors arise (and disappear).
Hopefully this one perspective is somewhat helpful.