One of the first SIP phones I ever worked with was the Linksys SPA941. This was a great for learning but with newer firmware releases this phone became worse instead of better. If you find a great deal on it and want to use for learning its still ok. It's not okay to deploy for new customers - the SPA942 is better choice.
The SPA941 is very similar to the SPA942. The only differences are:
1. No Backlit display
2. No PoE - You have to use the AC adapter (it is included)
3. No Dual switch (you cannot plug a computer to share network drop)
4. Terrible speaker phone
5. Low volume
The SPA942 shared the Low Volume and bad speaker phone with some of the releases. With the current firmware (as of this posting) the speaker phone and low volume for the SPA942 have been improved - but it still not perfect. You are not going to get Polycom speaker phone quality out of these devices - but you are also not paying anywhere near the amount for similar features.
For testing the SPA941 is good, for deployment use SPA942.