Just got my hands on a Cisco SPA50G, this is the revised SPA942 (probably the most popular voip phone)
The SPA942 was a very nice voip phone - it offered extensive features at a very nice price. There were some things that needed improving. The SPA504 fixes most of the problems. The one problem I was hoping it would improve immensely was the speaker phone but I am out of luck here.
Although with recent firmware upgrades for the SPA942 the speaker phone became usable its still was not a "great" sounding speaker phone compared to Polycom phones. Seems that Cisco just can't figure out the speaker phone.
The SPA504G has the following improvements:
1) Support HD/g722 codec
2) Handset - heavier - more polished -sounds better - sounds deeper
3) Buttons - have more of tactile feel - don't feel cheap anymore
4) Speaker phone - louder (no improvement is sound quality)
5) Sidecar - all the SPA phones now support the new and old 32 button sidecar - you do not need a "receptionist" phone anymore - anyone of them can be turned into one - all have the AUX expansion
6) Configuration Interface is more polished - looks modern (same options though)
7) Line/nav buttons and MWI are smaller now
8) Unit is a little heavier - seems more solid
Overall a nice improvement - if you don't need g722 and louder speaker phone your better off getting an SPA942 while they are still available since there is such a huge price difference.