In reply to laydnerf:
Well if you implement it well than you can skip the default do_pack/unpack functions.
As far as I know do_pack/unpack methods provides some policy classes, which adds some checks in pack/unpack.
Also using do_pack/unpack would allow you to use standard way of UVM coding, which is always recommended whereas using custom way.
I do not think it is much complicated to implement do_pack, unpack using predefined methods. May be your could share your questions here.
Thanks