It depends what kind of reads you're doing. If you're doing random reads, a good SSD will be faster than any hard drive on the market. A 146GB Seagate Savvio 15k.2 has a random read access time of about 3.2ms, an intel X25-E/M will have a time of under 100 microseconds.

You can see where SSDs make a big impact on desktop performance. Still, it's not nearly as big of an impact as they make in server storage. You can replace a lot of short-stroked 2.5" 15k SAS drives with a lot less SSDs or instead of storing everything in a 192GB of FB-DIMM (350W+) you can use PCIe-attached SSDs.