1. Grandstream - The company I love to hate. SIP's answer to DLink. Lots of features - good for testing. Terrible for customers. The stuff breaks or starts failing really quickly. Over time everything fails.
2. Linksys/Cisco SPA900/500 series. Good features all around. All the features work as advertised and quality is OK. The speaker phone could be much better and lack of BLF buttons keeps them from being deployed in many offices.
3. Polycom - Excellent speaker phone and call quality. Lack of message button and BLF buttons on 300 series phones keeps these phones from many solutions.
4. Snom - Good features, good quality, but real ugly. This is the volvo of phones. Practical but nothing to get excited about. Haven't looked into the 800 series phones but a huge improvement in the looks department
5. Aastra - Good mixture of features. Speaker phone good, call quality good, BLF buttons good. Variety of models from the traditional looking "office phones" to a more current look. The only real complaint about Aastra phones is the new models handset feels like it was designed for a little girl. The phones also have these funny little peg legs that can't position the phone vertical enough.
The one feature missing from all these phones with the exception of the Polycom 650/670 is end user call recording to usb keys.
This feature is asked over and over when talking to customers. Lawyers and Mortgage firms seem to want this feature more than anyone. If this feature was available I could probably sell more SIP based solutions as it provides a real reason and a business need to replace that aging key system.