Has anyone tried the Cakewalk MP3 Activation software at the Cakewalk store?
Is it worth it?

It is a convenience item. The license unlocks an integrated version of LAME 3.97. It is a one-time fee unlocking all future versions of SONAR.

As I still had Cakewalk Pro Audio 9, I already technically owned the MP3 license.  I had trouble running the activation tool on the Sonar X series, but CW tech support were excellent in sending me a new activation tool quickly.
 
But, to be honest, I like to export my audio as a WAV file and convert it to MP3 in another product.  I have an old version of Adobe Audition that I use.  There is just something I like about a simple stereo editor for trimming up the stereo mix, putting in the fade ins/outs and then exporting as MP3.

I just went the easy way and bought it.  It works great - it is the same one as one of the free ones out there, but as noted above, since the license is for forever - all future versions of Sonar get access to it.
 
I think it was running at only $9.99 the last time I looked - pretty good deal, in my opinion.
 
Bob Bone
 

It saves the extra step of exporting to another programs. You can of course set it up for free but for $10 who cares.

Whilst I haven't yet bought it, it is cheap - effectively just covering the license fee and the integration will slicken your workflow.
 
I think it bears mentioning too - Cakewalk have obviously thought about this ...
Rather than simply a money grab, splitting it out has obviously been considered as the fairest way to recoup the licensing fee - with a one time optional fee rather than having to load the fee onto every software product.

All the info is over at http://mp3licensing.com/

Probable reasons that the free/open source products 'get away with' not licensing are:
a. Fraunhofer/Thomson choose not to enforce arguably non-commercial product.
b. They are taking a gamble.

Obviously, Cakewalk, as a commercial entity, can't take that risk.

Cakewalk pays out the money for the licensing.
 
Bob Bone
 

robert_e_bone
Cakewalk pays out the money for the licensing.
 
Bob Bone
 

 
Yup ... so I was saying that's a fair way for them to pass it on. 

audacity is free and it has a lame converter that is free as well. 

+1 for purchasing it, for convenience sake. I bought it years ago for next to nothing, and it works automatically for each release of Sonar.