The cache should be able to help the drive with very small writes so that it can go to the cache and them be written to the drive. It can even be used to hold data that is being read.
With the right firmware, it should be able to say grab the current data set you are using if it is sequential and go off and do something else without needing to return to the platter until it has read all the data from cache.
For the most part the cache only tends to help with BURST rates. These do not translate into too much of a boost in day to day use.
You should look at SSHDD(Solid State Hard Disk Drives) as they combined a much larger(4-8 gigabytes on most) section of nand flash(same as a SSD uses) to allow often used data to be kept in this ultra fast section for improved performance. Even writes can be accelerated by sending them to this cache fist and writing them to the disk at a later time. This is also good because nand holds data when the power is out so it may be a bit safer if data makes it to the nand faster.
A review on such a drive. This is an older one. The newer ones are even better

http://www.anandtech.com/show/3734/seagates-momentus-xt-review-finally-a-good-hybrid-hdd/2