I've kind of answered some of this question in your OTHER QUESTION on this subject.
For reads SSDs are just great value for money - they're not the quickest (that would be memory or PCIe-based flash) nor are they capacious (such a the 3/4TB SATA disks everyone uses) nor are they the cheapest - but they are great value offering tens of thousands of random read IOPS for only a few hundred dollars each.
What they're no so great for is high (even middling) write applications, basically any given 'cell' (think memory unit) can only be written to a few tens or hundred of thousands of times before it dies. Think about that for a second, pick a bit of SSD 'disk' space, how often would something like a log file or DB write to that per day, without 'wear levelling' this space would be dead very quickly. 'Wear levelling' simply waits until the SSD isn't too busy then moves data from heavily-written-to space to less-written-to space - if it gets the chance anyway, most servers keep their disks pretty busy all the time.
As you can imagine if you have a very read-heavy application then SSDs make a lot of sense, same for laptop/workstation scenarios too where write load is low. For high write tasks you'll just be killing your SSD, even the best/most-expensive, very quickly indeed.
As for using them in R10 or similar, well it always makes sense to locally protect your data and R10 is a great way of doing this.