I actually never used it in all my years here, I prefer to go in person show your face and they remember you after a while, it certainly has helped over the years, it makes them be more lenient in my opinion, but also depends on your office.
you mentioned 90 day forms, the only form for 90 days is the TM47 for a 90 day report. If you are talking about an extension it is a TM7 form, and the TM30 is for reporting your address. If you could be more specific with your posts in the future, it will avoid confusion. Also you will only got another 30 days extension on your original 60 day visa exempt stamp, not 40 days and it will cost you 1,900 baht.
Takes 3 minutes to fill in the TM47 form and not long to lodge it, do it yourself, way better and cheaper than an agent. It is a free, so why pay someone else to do it for you.
You only need to do your 90 day report when you spend 90 consecutive days in Thailand, if you leave before that the 90 days start again from the day of entry which is day 1.