My understanding is that they run hotter and do not have the same efficiencies as other PSUs, which is odd because servers are extremely common.
Run hotter - yes. The HP supplies will get uncomfortably warm to touch @ full load but that is normal. They are made to run like that for years.
As for the eff -- they were made in 80Plus ranging from Bronze (pretty poor) and currently up to Platinum (excellent). Prices for the PSU's reflect that: 80Plus Bronze ones can be had for as little as $15-20 and prices go up from there.