I use the $5 servers for everything I run. Never had speed issues despite pretty heavy traffic loads, so I haven't bothered to upgrade.
Now, SSD is still slower than ram... way slower than ram. But it's way faster than spinning disk, especially on reads. I imagine there are some applications where SSD is fast enough that hitting disk is okay; you don't need to worry about having enough ram to cache all data you might read, and ram is still like eight or nine bucks a gig (vs. $0.6 to $2 for mlc ssd) - and if we're talking about reads here? MLC is fine for reads.
Of course, part of that ram difference might be that ram prices haven't fallen much over the last year or two, (in fact, most of the ram I bought last year, I bought at a lower price than the ram I'm looking at now.) while SSD prices have really fallen fast, so I don't know if this is the long-term price-equilibrium; still, I would expect ssd to continue to be significantly less expensive than ram.
Because of this, having enough ram to cache all the data you read often is extremely important to system performance when you are on spinning disk. (and having a pagecache that doesn't dump all the stuff you use often when you do a one-time read of a big file.)
I was suggesting that SSD might have fast enough read-access that for some applications, the penalty for reading from ssd rather than from pagecache might be small enough to not be a big deal, meaning you could get acceptable performance out of a system with less total ram.