The difficult part here to keep in mind is that Ping-O-Matic is not affiliated with the upstream services at all and so has no real influence on if they crawl the new content on notification or not and each services makes their own decisions algorithmically as to if they will or not.
For the pro's and con's...
- Easy to use
- Single endpoint to notify all networks
- Ease of integration
- Fairly good uptime
- No knowledge of backend process
- No feedback from upstream search providers on ping status
- No guarantee of real time or near real time indexing
- Single point of failure (If Ping-O-Matic goes down then all pinging goes down)
Additionally ping-o-matic focuses on pinging for blogs and so seems to focuson blog-related search services upstream and not so much general search engines.
Whether you use them or not is a decision that you as a webmaster need to make based on your own unique use case and needs.
Disclaimer: The pro's and con's listed are not an exhaustive list but an sample comparison.
Edit 1: When pinging whether you choose to ping the whole sitemap or an individual paage is entirely up to you. Functionally they both work just as fast as each other both in how long it takes to do as well as how long you see a result. I tend to find that pinging a single page after adding a single page works and pinging the whole sitemap works well when i have updated or added a number of pages. Con's wise they both share the exact same con, won't force a re-crawl, rather it will just encourage it.