Amitriptyline was a huge success for me and I am still on everyday to treat my IBS-D - it does a lot of the heavy lifting in my daily medication regime.
Required some patience and commitment to stay on it a few months and increase dosage - to a point where I was having side effects - but I would say within about 4-6 weeks even on 10mg daily I was already seeing a big improvement and after adjusting to a higher dose, it is still the most effective thing I have tried (of medications, supplements, dietary changes) in reducing urgency.
I went from having to run to the nearest bathroom (and anxiety leaving home/being trapped at a location for hours), postponing meals etc. to having plenty of warning and being able to eat more freely and go on with my day, drive, go into meetings without a worry.
I stepped up the dose over time in consultation with my doctor.
I was wary about longer term health risks and I didn't really want to be reliant on antidepressants long term. In terms of immediate side effects, I have found it causes nausea on an empty stomach and makes me sleepy, I was on 50mg a day which was quite high dose, made me just about fall off my chair about 90 mins after taking it (so if you do try it, take it before bed and do not drive), and felt a bit foggy all day and would start to fall asleep at work. It was not the type of antidepressant that makes you feel shiny and happy, it left me numb and very tired (but I slept like a log!). BUT that is not to scare you off just to let you know like anything, there are pros and cons, and it's about optimising what works for you.
it has been such a big part of getting my life back to normal that I ended up reducing my dose to 25mg (verrrrrry slowly) and whilst the effect is lesser now, I now have other medications in the mix which mean I have a pretty normal day.
Couple of other observations: In my experience double the dose does not equal double the effect (if that helps). And watch other medication interactions: just give it a quick google before you take something new or in combination with it.
So my experience was part of the solution (at least medium term) but not a cure.
Happy to answer questions if I can.