My view of Ubuntu
We use Ubuntu here, but by now I can't say that we're exactly fond of it. At this point we're using it not because we like it but because we lack a better alternative, which gives me a somewhat jaundiced perspective.
A disclaimer: this is about Ubuntu 6.06 (Dapper Drake, the LTS release that will be supported for a long time). Moving to more recent Ubuntu releases is not an option for us, because we need longer support periods than 18 months.
The good part of Ubuntu 6.06, and the reason we're still using it, is an unmatched combination of long term support plus a huge package selection. A large package selection is probably not important in 'enterprise' markets, but it is vital for us because our users want all sorts of software on the servers and we don't want to be building it ourselves.
(Debian might qualify, but there are two problems: first, it doesn't give me warm fuzzy feelings about having relatively current software when it releases, and second it would only have the long term support if it keeps up the current glacial pace of releases.)
Ubuntu's standard desktop experience is great, from the Live CD installation environment onwards (although the default desktop background is too dun coloured for my tastes). Lots of people around here are very happy with it, and with 6.06 you probably won't have to worry about OS upgrades for a good while.
The bad part of Ubuntu is that it only seems to work well for that standard desktop install. The moment you step outside of that, for example if you are installing a server, various wheels start coming off. Non-default or non-desktop installs seem decidedly second or third class citizens, regardless of what Ubuntu may say about server support.
(For example, while Ubuntu people like making worthwhile changes to how the system operates, they don't seem to push the changes through into packages that aren't in the default install, even if those packages are in the core. This causes me to live in fear of their great boot-time startup reform.)
Or in short: Ubuntu is slick on desktops, but stumbles badly on servers. Nor does it seem like this is going to change, since Ubuntu seems strongly focused on the (desktop) user experience.
The more I think about it, Ubuntu on servers seems like Debian with more current software (usually; Debian etch is probably ahead right now) and longer support, but also with more bugs and problems. And you get to run the same software on your servers as users are running on their desktops (assuming that your users stick with 6.06 on the desktop), which has various advantages.