/bin/su provided with coreutils is more functional than the current one we're using from the shadow package. I haven't given it a full testing, but coreutils su has been running on a couple of my boxes for a couple days now with no noticable problems. All of the <recent> other distros that I have access to also use coreutils su binary.

[root@rock root]# su -s /bin/bash dale
[dale@rock root]$ exit
[root@rock root]# pacman -S shadow
:: shadow-4.0.4.1-1: is up to date. Upgrade anyway? [Y/n]

Targets: shadow-4.0.4.1-1

Proceed with upgrade? [Y/n]

checking package integrity... done.
loading package data... done.
checking for file conflicts... done.
upgrading shadow... done.
[root@rock root]# su -s /bin/bash dale
[root@rock root]# id
uid=0(root) gid=0(root) (..shortened..)
[root@rock root]# exit
exit
[root@rock root]#