Thank you for your query - to make life simple...here are the major differences.
1) ThinkPost offers a full-featured webmail (which is not important to some)
2)ThinkPost is based on funambol, but the software is tailored to our needs and servers
3) I am always sketchy about free services due to adequate backup and hosting reliability. We have enjoyed 100% uptime for the last few years (our servers are hosted at RackSpace - the only privately managed Tier 1 datacenter - comparable to IBM / AT&T Datacenters)
- and then there is the issue of duplicates - one that plagues almost every sync offering. For us, we take the issue very seriously so we have seen a large reduction in the probability of duplicates over the last year - to the point where it almost never exists. Of course, there is the odd time and it happens for whatever reason, but we get it fixed, quickly.
It really depends if $4 means a lot to the user. For that $4, you get reliability, a quality program, and fastidious support.
Much like email hosting nowadays - if one can't find a reason to pay for email, then there are free alternatives.
The general mantra is...you get what you pay for =)
Thanks for your inquiry, hope this helps answer the question!
Sync your Contacts/Calendar/Notes/Tasks wirelessly from your PC to BB - http://www.thinkpost.net