I haven't used it, but I rec'd the following email from someone that did:
In an effort to get ahead of the crowd I decided to try to do my 90 day reporting before the end of July and went today on the 16th. I tried the QueQ. It worked better than I anticipated in that it gave me a queue number that was current as of the time before I left my home. Anyway I proceeded to Immigration, showed them the number the app had given me and I was directed to counter 4. Everything went smoothly and I was in and out within 5 minutes.
If folks need to get anything done at immigration there are very few people there requesting service right now. It might be a good idea to go before the 31st of July.
In response to my reply to the sender regarding the number he received, he sent the following noting it was on his phone, which he showed at Info desk:
I think what the program gave was a standard queueing number. I think it was 4B008. That would correspond to a number for desk 4. The folks working the information desk seemed familiar with it.
I just showed the number on my phone at the front desk and at the reporting desk.
I don't recall if the Queue tickets you get at Immigration have the "B" in the sequence - if not, they may denote it as having been obtained online. But, if it gives you a regular number in sequence, i.e., the next number to be issued, it appears it could get passed up for other numbers they issue if you get it very long before you arrive.
Since he was doing 90 day report and they are generally sitting there twiddling their thumbs at the moment (my experience when I did my report last month at the time I picked up my passport after getting my annual extension), time of arrival probably doesn't matter even it it was a regular queue number.
I will still be interested to hear the experience of someone using it and going in for a service other than the 90 day report.