As a rule, if the price is below € 300 there is something missing.
I am not an expert on HighPoint but this one looks like a hardware raid controller - but there is no batteries protected cache and there are only 2 internal connection - so you are limited to RAID1 (simple mirror with 2 sata drives) and operation without cache which gives bad performance.
Batteries backup for cache is very essential in case of a power loss. But the batteries can also fail as they have limited life time and problem if the server is getting hot.
Adaptec presented a controller with flash memory backup instead of batteries - which looks more reliable. I have no experience whit these newest technology but it looks like a solution for the "dead" batteries problem.
Starting to think its not supported. I went ahead and purchased this card and right after I press enter on the proxmox splash screen, i get the dreaded blank screen of doom. Debug does not change the result.
If you really like to have a good linux supported hardware raid controller - choose the products from http://www.3ware.com/ . the are not cheap, but you wont have any problem for a long time. Before I moved to those controllers, I struggled a lot with various other vendors. I dont have shares of 3ware - just my experiences.
cheers
tom