[JAM, MDA Compact, S100 General] christopherwoods:
Yes, it does share the same ground in terms of functionality, but there are some very useful little features (including some things which I think are shockingly lacking in WM2003SE by default). Let me elaborate with a screenshot and some explanation:

It adds the section to the Today screen (which you can see below my Calendar). You have two tabs, Main (you can't change these icons), and Applications (you can add whatever you want to this tab, but the section doesn't expand like SPB Pocket Plus to accommodate more icons, instead a small down arrow appears at the right-hand side to indicate there's another 'level' of icons, clicking the arrow kinda scrolls down through the list, showing the next row).

Where you see the word "Meeting", if you click it, you're taken to the Sounds & Notifications options, where you can choose from the following sound profiles:

Normal

Silent

Outdoor

Meeting

Loud

User Define 1

User Define 2

User Define 3

You can edit any of these profiles, obviously O2 are suggesting a few, normally I use Normal but I just used Meeting to illustrate the ability to choose.

The little red battery icon you can click to go to the Power control panel applet, and clicking on the small memory chip icon will take you to the Memory settings control panel applet.

Handy, Otherwise-Missing-From-WM2003SE Feature Number 1:

O2 Plus adds a "soft reboot" button to the Applications tab (it's a circular orange button with an arrow going almost full circle around it, when you tap it it asks if you want to soft reset, and if you press yes, it does it straight away. It also has a backlight setting button, a Beam setting button, a File Manager button, and a VCard button. Clicking on the (very) small O2 icon on the top-left of the O2 Plus Today section takes you to the O2 Plus preferences, where you can do further stuff:

Handy, Otherwise-Missing-From-WM2003SE Feature Number 2:

O2 Plus adds the option to add a "Save number to contacts" option, taking away the need to get the number from your call logs / the top of a text message, copy it to the clipboard, go into Contacts, make a new contact, write the name in, and paste the number. If you're reading a text message from an unknown sender, you can just tap and hold on the number and it'll show a "Save Number To Contacts" option, which is VERY handy. You have to enable this feature in the options, but once it's there it stays there.

Handy, Otherwise-Missing-From-WM2003SE Feature Number 3:

You can add notification icons to the bottom taskbar. I've chosen to show the GPRS On and Call Forward On 'Indicators' (the orange phone with four buttons next to it is a shortcut to the Call Divert options, I'll explain that further below), and it adds its own O2 icon as well (you can also get to the O2 Plus options by clicking on that icon at the bottom too, and choosing 'Options').

The Shortcuts options in the O2 Plus preferences allows you to add icons to the Applications tab on the bottom taskbar: I have File Explorer (oops, don't really need it but I had it checked anyway, as it's also on the Applications tab on the main O2 Plus panel), Call Forward Profile, Sound Volume Profile and the Notes options checked, hopefully you can see where they've been respectively placed on my screen.

The Others O2 Plus menu offers these choices:

Vibrate upon receipt of call response

Enable date display

Enable call divert profile

Enable menu option to save SMS sender to contact

Attempt to connect MSN Messenger after phone call

Attempt to to connect MSN Messenger upon establishment of a new GPRS connection

The Applications preferences tab allows you to add your own icons to the Applications tab. I added an icon for my Voyager installation, the software adds its own default icons (which are pretty useful shortcuts, you can always remove them if you don't like them).

Then there's an About tab, complete with seeo2.com web site address. ;)

I particularly like the enhanced call divert features, the ability to quickly switch between sound profiles, the ability to add more icons to the bottom Today taskbar... All very useful, and for the price of free, who's complaining? It does many of the things SPB Pocket Plus offers (though Pocket Plus is also useful for its Pocket IE enhancements, clipboard enhancements, plus all the other excellent goodies the software brings with it), I have SPB Pocket Plus installed for those and I've removed its overly-cluttered Today menu pane, and replaced it with the O2 Plus Today pane. The software has also managed to consolidate the battery and free memory readings which I used to use the "O2BatteryIndicator" plugin for, I don't need that know as it's redundant! :)

One of the more useful little tools to come from O2.