Reliability up in the air
Drive capacities aside, SSDs have two obvious advantages: speed and reliability. But realistically speaking, the speed factor is hampered by capacity. Allow me to explain:
Pretend that there are two identical machines, one with a 128 GB SSD and one with a 1.5 GB SSD. Say also that there was a collection of movies that equated to roughly 1 TB of data. Because you're unable to fit all those movies onto the 128 GB SSD, they would have to be burned onto DVD-R discs and stacked and stored. Keep in mind that both systems are limited to the drives they have--one SSD and one HDD. To watch the movie you want, you may have to scour through your stack of DVDs to look for the ones you want. This takes time and gets factored in to overall "access time." With the 1.5 TB drive, everything is at your finger tips for access, movies or otherwise.
Naturally you can also add a big HDD to accompany the smaller but faster SSD to improve total performance. But this just lends credence to the fact that HDDs will remain around.
In terms of reliability, there's been much debate about just how reliable SSDs are. There's a stigma over SSDs: that memory sectors will fail over time with repeated erases and writes. On average, SSDs are rated at roughly 100,000 write cycles per memory sector. Manufacturers claim that this roughly gives an average SSD a 1,000,000 hour MTBF rating.
Some users are concerned with using SSDs as their main operating system drive, which is what they should be used for anyway. Unlike archival drives, system drives take the highest level of beating. Even so, I reckon that the typical SSD has a lower failure rate than HDDs due to the lack of moving parts. There's a substantial increase in the chance of failure for HDDs due to the myriad of mechanical parts. Having many memory sectors fail over a long time is a lot less damaging than is a single head crash. In most cases, a catastrophic head crash--where the read/write head actually breaks off the actuator--your HDD is completely hosed. Even with a 50 percent sector failure in an SSD, the chances of recovering data is far higher.
Where SSDs will succeed first is in the mobile market, where reliability and stability are heavy factors. But until prices become more mainstream, expect to see SSD laptops remain at the upper echelon of premium-priced configurations. For the desktop, the HDD will remain the king of storage for several more years.
Those who were looking forward to 2009 or even 2010 to be the year of SSDs, you'll just have to wait longer.