I had it only once for a business trip where only Y was available for departure and arrival
I was told as it was the first time for the company, it was only valid for one trip. We also have just one small BOI company with around 250 people.

So I guess if your company is a large one and your local staff used to get this from BOI, they might get a mutiple one.

I'm not sure which details are being checked as I haven't submitted a card with any details this year; they've all been entirely blank save the airline's code. I have heard that the immi officers are checking the airline's stamp against what is filled out on the immi form (which hasn't affected me) but beyond that I've seen no evidence for the claim.

you are probably right that the immigration officers do not check much Details, but the dragons get more and more nitpicking

I'll never try and find out. On TG I've always got fast track included with my ticket, but in other cased like one class PG flights, I'll continue to buy fast track. It's probably likely that my old unused passes would work, but immigration officers in any country are officials that I would never want to piss off.

That's very much a YMMV situation. I've only been hassled outbound and only once that I can recall.

but there are dragons at the entrance outbound and inbound. Maybe they do not dare to hassle you as you look like a dragon slayer!
anyway, should I recommend a fellow board member to try his luck with old non valid vouchers? I even still have a bunch of purple fast track vouchers from TG. Should I try my luck with them?

Well we come to the board to share experiences. I don't take those experiences as recommendations but rather as data points for me to make my own decision. I'd never come back and blame FlyerTalk for bad advice when things go south.

As for your old purple ones, so long as they the same as those that are currently being used. Now and they they do change the format.

I've never had a peep from any dragon inbound; rarely have even seen one at the entrance. Outbound is an entirely different animal (dragon) and they are hyper vigilant to check each pax for entry. But, again, only once in many years have I been told to fill in my info prior to entering the area. None of the airlines I've received a pass from recently, the preponderance of which is PG, fill anything out so I'm almost always showing up with a blank card and, save that one time, all is fine.

Two years ago I was trying to burn through some older UA cards, after the airline pulled out of BKK, and there was no pushback. Now it seems the agents are checking the flight against the card's issuer so I wouldn't attempt it. But the difference between the old TG purple vouchers (which are no longer accepted) and a two year old CX pass are like night and day.

Not that I'd recommend tyring your luck with the old purple vouchers but if you wanna try I've got a handful I'd be willing you give you gratis.

no need! I have also a bunch of yellow premium lane cards which i might use if I have no fast track access (by TG platinum card or J arrival) and the lines are long...
thanks anyway for your offer

sure you are right. just made my recommendations based on some 100+ arrivals during last couple of years. the purple ones are certainly not valid anymore as TG stopped to give out those many years ago.

Well for the first time ever, I had a wait. Outbound. Only 3 in line in front of me when I started, but one had an extended family who kept joining the line (in front of me). Total CF. They eventually shut the line, and I had to start over at the back of another line. Got to the counter just as a senior "supervisor" was taking over. Don't know if it was incompetence, orneryness, some misguided idea of how to set an example, or maybe just plain dementia, but it took her over 5 minutes to check and stamp my passport.

Took close to 20 minutes in total. I've been in and out a couple of hundred times and it's never taken more than 2 or 3 minutes before.

you mean in Premium Lane or TG Fast Track?
for the latter there are usually less then 3 in line...
but even such incidents once in a while do not bother me. we are still much better off then those in the normal lines

Is this (TG Fast Track) still available for incoming international flyers for Business Class passengers? Any recent experiences?

Yes. I used it in December. It is actually a ‘priority’ lane for Monks, Diplomats, crew etc. It is a ‘tape lane’ to the far right of the main immigration flow, and to the right of Thai citizen smart gates.

There is a different thread also talking about this where I posted a picture showing where to walk.
https://www.flyertalk.com/forum/36502770-post317.html

indeed!
sometimes hidden and sometimes, they try to channel all foreigners to the main lines... I usually just keep walking if some staff tries it on me...