I use the Verizon Network here in the US, but the phone has a SIM card slot that is easy to unlock (just call Verizon) and when I'm overseas I can use my overseas SIM cards. No longer have to juggle my US phone/PDA and a second phone for talking - one phone can control/contain all my contacts.
Definitely does everything your Axim 51v does, and of course can tether to a laptop. Heck, I run WMWiFiRouter on mine and it becomes a WiFi hotspot with two screen clicks. It is my sole access to the Internet; right now it's my WiFi router at the house, sitting in my office as I'm typing on my laptop in the living room.
HTC TouchFlo 3D is a great UI for the phone, but I got hooked on SPB's Mobile Shell a year ago and still use it. Amazingly customizable UI manager, great tools built-in, makes the phone easier to use and configure than the iPhone.
And it still has the ability to natively edit Word, Excel, and PowerPoint files - it is Windows Mobile, after all! I even played with the TV out (optional $20 cable) - I can show my 640x480 movies on any TV, and even display PowerPoint presentations from my phone. VERY slick, very cool!
And yes, it does play audio files - the 16 GB microSD card I have installed right now has about 1200 songs, a dozen movies, and 60 eBooks (which read very nicely on the big, bright screen).
i got the HTC G1 about a month ago and love it, touch screen and full keyboard, does everything, only issue just waiting for a java app