I have the HS850 an LOVE it... Now for the 7520 (I have a friend who has this one) the ONLY one that I now works well is the BT250 by Jabbra.
The HS850 works GREAT on the 7290 (what I use it on). My friend bought an HS850 with me, and wound up selling it because of echo...
Crackberry User and Addict.
7250 on Verizon w/EVDO and 4.0.2 Update (Active)
7290 on T-Mobile (Retired)
T-Mobile 7230 (Retired)