It's good at being a gateway between your office LAN and the internet. It's especially handy if you have two internet connections for redundancy--it can route traffic over both. You can install a content filter to restrict what inappropriate stuff gets weeded out, hours of connectivity, and so on. You can have traffic scanned for viruses/malware. You can have a centralized file server and print server. You can have an FTP server.

I see it as a way for a small business to get access to the tools that any company wants to have these days, at a very reasonable price. It lowers the level of tech skill required in-house...someone should still know something about linux, but you don't need to be a systems guru. Hiring outside firms to do all the things that ClearOS can do is possible, but would cost a lot more.

Define "best" - you are only going to get very subjective answers... what is best for you may be the worst for me, even for each application within the overall distribution.

As Malcolm hinted at in his reply the answer depends on a number of factors.
Type of user - home, soho, small company larger company etc
The requirements for that environment
The administrators knowledge, experience and abilities. Even in a home somebody has to install and hopefully look after the system.
The hardware/budget available
If applicable the company's policies re IT that must be complied with such as security, monitoring, access etc
The level of support required from outside the home/organization

All-in-one firewall/gateway/mail server/webserver/file sharing/vpn/media server solution that runs on cheap hardware.

well, that's how I use it anyway!