SSDs are very fast, quiet, and cool (as in temp), so many people prefer them for running core programs and the operating system, primarily due to the faster response time. However, SSDs are very expensive compared to mechanical hard drives, with anything over 256GB or so being prohibitive in price for most people. Most people like SSDs somewhere in the 60-128 GB range, enough for OS, a few core programs or games, and headroom.
So, given that most people have more than 256GB of stuff, you need a place to store them. This means that people are also buying larger HDDs, 1-2 or more TB, to use as storage drives. This holds non-essential or core programs, music, documents, video, etc. These are files whose performance is generally not improved noticeably by an SSD, so there's no real dropoff in speed or ease of use. The same goes for games - in the great majority of cases, having a game on the HDD as opposed to the SSD makes no performance difference.
So there you have it - fast, quiet SSDs for OS and core programs, HDDs for storage of bulk files and most other programs.
i personally will never buy another 128gb SSD, 240gb is the sweet spot but its expensive and isnt essential to a PC. the 200 bucks could be spent on a better CPU or GPU, but for more people once you have a SSD you never go back
I use a 256gb SSD for my boot drive but HDD for every thing else. With over a 1tb just in my Steam folder I could never get a SSD for everything. But really the performance gain with a SSD is limited to booting the system and the core OS anyway. Along with my SSD I have 5 3tb SATA III Seagate HD drive's and between my movies games and music it is almost not enough space!
Cost vs performance.
As All Have indicated, the SSD is About 40-> 80 Times faster than a HDD.
However; That said, and SSD will:
.. Considerably improve boot time - that is going from "start loading OS" -> able to open first program. Also shortens shut down time.
.. Speed up program load times - How noticable depends on program.
.. Will load any files much faster than a HDD, providing the file is on th SSD.
.. Speed up web surfing, downloading files from the web, nor speed up email
.. Will Not make programs RUN faster. Does not improve FPS in games (Will speed up map loading providing the map is on the SSD.
.. Will NOT speed up file load times if file is on HDD
Added cost, since a HDD is still need, is about $100 for a 128 gig SSD, or $170 for a 256 gig (on sale). There are cheaper version but generally do not recommend.
.. Currently the Min size that I recommend is the 120/128 gig SSD. I DO NOT recommend the 60/64 gig SSDs
.. Generally Windows 7 OS + programs take about 35 Gigs. Add to this any games you plan on installing.
.. For a 128 gig SSD. (1) Formated size is about 119 Gigs, Difference in way manuf count vs the way a computer counts. (2) Then you MUST leave an amount free so that Wear leveling, Garbage Collector (CG), and Trim can work here magic to keep the drive near manuf performance specs. This started out at 10%, then jumped to 15% and now the recommend space to leave free is 20->25%. This Now cuts the usable space down to approx 90 -> 100 gigs Available (depending on 15% vs 25 %)
Ref on "free space": http://www.anandtech.com/show/6489/playing-with-op
I have In-use two laptops and three desktops, ALL Have SSDs + a HDD except one laptop - Would Never go back to just using a HDD.
Bottom Line, if You can afford the extra $100 go for SD + HDD. Remember the HDD is the single Biggest bottleneck in system performance.