Looking for a smartphone to replace my Motorola L7 and Palm V combination, I bought this phone because it had (1) WiFi, (2) a QWERTY keyboard, and (3) all the good cellphone features I appreciate from Nokia (battery life, call quality, etc.). First, let me say that I had to return it because of a problem with my home electricity - it seems that the european multi-voltage charger couldn't generate enough voltage off my low 109VAC to charge the phone. This has been reported by some others, but is apparently pretty rare. Unfortunately, it was a deal-breaker for me. Strangely enough, the phone did the initial charge fine, but would never charge again.
But in the 4 days that I used the phone, I was quite impressed. It really is an outstanding product, and the only thing else on the market that has everything this has (and more, in fact) is the HP iPaq 69xx, which is too new to know much about. I would generally agree with the glowing reviews already written, so I won't repeat everything they've said - it's pretty much true. But coming from a Palm PDA experience, I was somewhat disappointed by the PDA capabilities of this phone.
For example, there is no search capability! OK, the File Manager allows you to search material that's under its control, but that doesn't include the phone's contacts, notes, and calendar/ToDo apps - the very things you need to search! There's a decent search application from sept-solutions.de, but it can't search the notes - a serious limitation for me.
Next, the sync can't use many of the phone number fields from Outlook. For example, I sync'ed my Palm to Outlook, and many entries use the "Main" phone number. Outlook got these fine, but the Nokia PC Suite couldn't sync them to the phone. A real nuisance. A simple remapping setting in sync could eventually solve this, but for now, I'm out of luck.
Next, there are no categories for any of the phone's applications. The very nice AquaCalendar 3rd party application allows you to assign categories to calendar items (including tasks), but these aren't transfered in a sync with Outlook. Personally, I can't see how anyone could survive without categories for tasks, notes, and contacts (not to mention calendar, but Palm doesn't have that, either). The E61 has contact groups that are essentially categories, but these don't seem to get transfered in a sync.
Another problem I never found a solution for is that my Palm has several hundred items in the ToDo list, all undated. Outlook has undated tasks, but it seems the E61 does not - all were assigned to the day I sync'ed. And there were too many for the calendar app to handle without getting memory full. So basically no way for me to transfer my ToDo list!
Yet another problem is that you can't attach notes to calendar, task, or contact entries. This is a deal-breaker for me. AquaCalendar does allow notes ("descriptions") on some calendar entries, but it's not nearly as useful as what's on Palm.
Another problem is that when you want to change the date of a calendar entry, or simply go to a date that's a couple of months away, you don't get a nice month view the way you do in the Palm's calendar app. It's quite a nuisance to have to scroll or guess and correct when going to or setting a date that's far in the future.
Finally, while this is not another show-stopper for me, I use PocketQuicken on my Palm. This outstanding app is also available on Windows phones, so should work on the iPaq, but I haven't found anything this good on Symbian or J2ME platforms. A very nice free app, JabpLite, seems like it would do the trick, but I've never managed to get it to read QIFs from Quicken other than by very slow bluetooth file transfer.
In summary, if you want a truly outstanding smartphone (without a camera), the E61 could easily be your best choice. But if you're expecting the usability of a Palm-based device, you'd best look elsewhere. The Treos might make a good choice, but they don't have WiFi (yet), and as a phone, they can't touch the Nokia. As for the iPaq, it might do the trick, but it's somewhat larger than the Nokia. Motorola Q? Nice, but also no WiFi. I haven't been able to check out everything from HTC, yet, but the last time I looked, they didn't have everything I was looking for. The perfect phone still doesn't exist, but the E61 sure has raised the bar.