Earlier this year, eBay won an award from MySQL. This was for application we built that we originall Gem Cache. It is a caching tier that is built on top of MySQL. When the caching tier was designed, memcached was given a lot of consideration, but there were some very good advantages we got out of using MySQL instead.
First off, can MySQL be as fast as memcached? Absolutely. MySQL is aggressive about keeping things in memory, and if everything is in memory, it will be as fast as memcached. You can use MySQL's MEMORY engine, to accomplish this, or you can stick with MyISAM and let MySQL's caching put things in memory for you. Obviously you need to split your database, but we already knew how to do that efficiently. With that in mind, here are the advantages that MySQL offers.
1.) SQL Semantics. You are not limited to just simple "put" and "get." You can do selects and joins, aggregates, etc.
2.) Uniform Data Access. Do you use some kind of ORM? You can leverage this with a MySQL based cache.
3.) Write-through Caching. In a typical memcached setup, updates are still done to the database and this invalidates one or more objects in memcached. With a MySQL based cache, a row in the cache corresponds to a row in the "real" database. So we can write the cache and then asynchronously update the system of record.
4.) Read-through Cache. Similarly, you can always attempt to read from the cache database. If there is a cache miss, you can invisibly read from the real database and add to the cache at the same time.
5.) Replication. MySQL allows for replication of data, so it is easy to add redundancy and fail-over to your cache. Replication can also be useful when you have multiple data centers.
6.) Management. There are lots of great management tools for DBA, operations folks, etc. to use with MySQL.
7.) Cold starts. When your cache is a copy of database rows, it is easy to bootstrap it from your source, since the source and the cache are so similar.
8.) Eviction. Memcached gives you basic expiration, but otherwise you are handling eviction yourself. The caching in MySQL is a more useful LRU policy.
So there, just a few obvious advantages to using MySQL as a cache instead of memcached. Now I know that a lot of folks use MySQL as their "real" database, so it may seem weird to use it as a cache as well. But they are probably (hoepfully) using InnoDB for the "real" DB and that really is a different beast than MySQL with MyISAM or MEMORY tables. And it's not like you have to pay for extra licenses are anything... What are the advantages of memcached over MySQL? The only obvious one to me is if you want to cache things that don't fit in the database, like deep object graphs or HTML fragments, etc.