I am an Administrator at a university business school.
We have been using Notes Version 4 for a few years, a reliable, stable platform.
We have implemented R5 recently, here's a few observations.

R5 needs considering at the server and client ends separately.

At the server level, there is a new Administrator's interface. It is loaded with more functions, and allows easier access for setting account details, creating users, moving accounts etc. A clever feature.
The actual server install is similar, but there are lots of new options, especially regarding Internet mail, to use.
A word of strong advice: don't rush into using R5 server, research how it uses native SMTP and such like, a real change from R4's methods!
Overall, it is good, and makes Admin easier and more efficient.

At the user end, the client I have mixed feelings about.
Lotus have redisigned the GUI, using a bookmarks and link interface, with a feel similar to a web interface.
To be honest, the idea is good, the execution a little ropey. It is easy to wrongly set up a client, and spend a good while finding problems.
Also, when an account is created, there is now an Interent Address field to fill, make sure you are completely aware how this works for external mail, it is a dog if you do not set it up correctly.
As with other Notes versions, it also uses a lot of space on PC, and R5 mail files seem to use a lot extra.

Overall, as with previous versions, a good product once acclimatised, but a nightmare if wrongly set up, and seems hard for MS Outlook users to understand!!