Hi electrokawal,
I use HPM-82/Fontopia combo with my K810i. I bought the HPM-82 handsfree for Rs.1500/- from Sangeetha (in Forum Mall, Bangalore) on September 2007. I don't remember the Fontopia model (it came with a carrying case also). I bought it for Rs.600/- from Sony World some time in August 2007. The main reason I prefer Fontopia over the ones which came with HPM-82 is because, while travelling through a bumpy road, the in-ear type ear phone which came with HPM-82 comes off from my ears very frequently (I tried with all the 3 plugs which came with it). Also, I feel that Fontopia has better bass and treble compared to the HPM-82 ones. I have been using this combo for the past 5 months, regulary sleeps with them, still no problem.
One thing I would like to warn you is to check the handsfree thoroughly. When I inserted the new handsfree for the first time, my K810i didn't detect it. So, I immediately went to the shop and got it replaced with another new one. This one worked fine for the first 1/2 days. Then, during 50% of the time, the phone stopped detecting it. ie, when I insert it, not while listening to songs. I am sure this was not because of any loose contact, may be the weather was causing it. I also remember that this second one was not detected any time when I used along with the charger. Then I went to Sony Service Center and got a (3rd) new one after 3 weeks of waiting. This one has been working fine ever since.
By the way, which phone did you buy?
Edit: Came to know from the other thread (poll: K790 or K810) that you bought k790i. Good choice.