Have you tried the User/ OU/ AD Group Relations from within the asset page itself?
Asset-> Select Asset-> Edit Asset-> scroll to "User/ OU/ AD Group Relations"
From here you can modify the assignment date, its type (we use "Used By") then use the "Select user, OU, or AD group drop down item and assign it to the user.
It sucks that it can't be a mass edit field, but it will allow you assign assets and view assigned assets form the user page.