Got reply from KMD, maker of the Free sms module..

" Hello,

There are no ads, or sign ups or Any hidden fees.
...
"

I believe him fully, and now recommend this for some types of use!

The point by huston is the reason..
You will not be sure messages are sent - so then it is good for any use where users are ok with some sms not arriving.
You also absolutely have to get the provider right for the user to receive sms.
(I expect it will work about 80% of the time or more - loose see more guess)
So letting users know this, will avoid sour grapes in support inbox :)
Apart from that, this is a really clever module! ;)

*Think* I fugured out how this is done :)
Did not work for my phone though, probably cause I have blocked this option on my phone provider..

If we did not already have sms systems, I'd install this for some fun and user attraction.
(But would have to watch for spamming in outgoing traffic log - even block words, senders, receivers etc..)

Cheers :)