SSDs can give substantial improvement for (almost) any system using slower storage devices except memory, in fact it efficiency with MLC memory used would be greater for portable low-performance (relatively) systems. You can even replace HDDs in some HDD-based PMP to get some better performance and shock resistance.
Still, for “middle-end” SSDs make very little sense for now. Probably in a few years they will be worth buying though, after a price drop over unreasonably high priced SLC NAND (“MLC” has only 2x capacity over SLC, but still SLC products are usuually over 2x more expensive) and appearance of alternate technologies with greater write endurance (not before they return their money invested in NAND – that is one of the main reasons for not introducing “better” solutions). As of now, NAND doesn’t seem like “memory of the future”, with its low endurance and failure rate.
In other comments i didn’t mention also one more “bad” thing about NAND memory – its failure rate. The failure rate increases greatly from SLC to other xLC each time 1 bit of capacity is added, thus requiring more error correction circuits for each “next generation” (starting from 3xLC (TLC) can be used probably only for rarely rewritten external media, possibly as a good replacement for optical drives).