What is the quality of SSDs included on a Mac? Are the SSDs used by Apple high in quality in terms of Speed and longevity (which is a particular issue with SSD). If not would you recommend buying a separate SSD and installing it after purchasing a mac?
Apple uses a mix of Toshiba and Samsung SSDs in the current lineup of Macs (pre-2011 they used Toshiba only). Neither are the fastest drives around, but they're still pretty speedy, and in most day-to-day cases you wouldn't notice the difference between a Toshiba or Samsung and the fastest Sandforce, Intel or Crucial/Micron drive. The reason for this is that almost all SSDs are so much faster than a traditional hard drive that the relative difference in SSDs is fairly small.

The Toshiba and Samsung drives are both respectable performers but trade that extra x% that differentiates the top performers for better reliability. From Apple's perspective it's a pretty smart trade, as the perceived performance in 95% of cases is still great, and the long term customer satisfaction is more important. For more specifics see the SSD section of Anandtech's 2011 MacBook Air review.

As to whether you should get a 3rd party drive or an Apple one, here's my advice:


If you know you need the best possible sequential read/write performance for a specific purpose, get a 3rd party drive. If you're not certain you need the extra performance, you probably don't.
If you're going to be getting a small drive (~128 GB or so), go with the Apple one. In a MBP you can get a 128 GB drive for $200, which is pretty competitive with prices at NewEgg, etc.
If you're want a bigger drive, you can generally save yourself ~$150-200 by going with a 3rd party 256 GB drive, but if that money matters less to you than reliability (and the reduced hassle of having Apple warranty cover the drive), get the Apple one.