Geekzone has published a very extensive review of O2's XDA Exec (HTC Universal). Although the O2 XDA runs Windows Mobile 5.0 on an Intel CPU running at 520MHz, the reviewer says the device seems slow. The XDA Exec does have a lot of bells and whistles so check out the review for the whole story.

"How versatile is it? Well for starters it will connect to a standard GSM/GPRS network so you can have voice and data directly on this device. If your needs claim for a faster mobile connection, then it will connect to a UMTS network up to 384Kbps (here in New Zealand I am using the Vodafone 3G service). But if your costs are getting too high when using this cellular connection then you will find the option to connect to wireless LAN networks appealing – and fast, with 11Mbps connections."