Yunchang, wow, that *is* ULTRA low! I was on low dose for basically a decade, with regular breaks (about a month to six weeks each year). I was taking about 40-60mg each week (so, let's say 50mg per week on average), usually in 10mg or 20mg doses every second or third day. I didn't have any side effects except for almost clear skin (I was still getting breakouts around my period, but nothing I really worried about). At that dose, it would take between three to four years to match a single "traditional" course of accutane, so you could say I've been on three courses.
I was side-effect free on this regimen until this year. About six months ago, I noticed mild hair loss and significant crepitus in my knees (crackling and popping sounds, but no pain). The hair loss is now subsiding and could well have been the result of a very stressful event last summer, and the crepitus is common in my family, so these could be totally independent of accutane, but I've stopped accutane as well as running, and am working on strengthening some muscles supporting my knee cap. After stopping the low-dose accutane, I was on doxy for a few months and then got sick of how finicky it is (you really have to be careful how you ingest it so you don't give yourself an ulcer, which I did, multiple times -- oops). Now I'm on 25mg of spironolactone which is keeping me almost entirely clear. I had taken spiro in my early 20s and needed 100mg per day, but now this small daily dose of 25mg seems to be enough.
I wouldn't recommend low dose isotretinoin over such a long term to anyone without proper supervision and blood tests. It is not a well studied approach, and risking musculoskeletal or other unknown systemic consequences is simply not very smart, to put it mildly. However, it would indeed be interesting to see some serious attention be given to this approach by the research community.
Yunchung, what does your doctor say about your approach now that you've been on it for a while? Does s/he still dismiss it as laughable? I must admit that dose sounds too low for your body to even register it, but if it's working, who are we to argue?