I've been running the Nokia Email Service on my E51 for a couple of months now. It's definitely the best email client I've tried on the handset - slicker than the native S60 email client.
Rather than having to set up yet another email account and forward your email to it, the Nokia Email Service collects and sends email using your existing email account. It's currently in beta and seems to work well. I use it to collect personal email from my Gmail account.
A couple of points worth noting about the software. I started with the sync set to email push and that killed the battery. I've now got it set to 1 hour and get much better battery life. Nokia E Series handsets support Active standby plug-ins where you can have multiple applications displayed concurrently on the home screen - e.g. Missed calls, New message, Calendar, Inbox. One disadvantage of Active standby plug-ins is that only one Inbox can be displayed - SMS or email. However the Nokia Email Service creates a separate Active standby plug-in on the home screen so I have both my SMS Inbox and my email Inbox displayed. A nice enhancement.
Currently the software is a free download and is available for most N and E Series handsets. Nokia has yet to announce what will happen when it comes out of beta, in terms of handset support and whether it will continue to be free.
Nokia Email Service is definitely on my list of 'must have' S60 apps.