Most customers like to use SquirrelMail as it is "simple" and sometimes simple is good. RoundCube is great, we simply provide Horde, SquirrelMail and RoundCube at the same time and let the customer choose which one to use.
John Hayes Virgo Hosting
Quality yet Affordable Linux Hosting since 1997
Personally i love RoundCube when i'm using cPanel, i find it works really well.
SuperBytes.net ~ US/UK/CA OpenVZ VPS Hosting
TinyVPS.ca ~ A Tiny VPS for a Tiny Price!
FanaticalHost.com ~ US Shared and Reseller Hosting
Matthew Hall ~ Kerplunc Hosting ~ matt [at] kerplunc.com