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:
I dont know the exact drives Apple uses, but I would say that when buying an SSD, its good to take a look at what Jeff Atwood had to say on the hot/crazy scale: http://www.codinghorror.com/blog/2011/05/the-hot-crazy-solid-state-drive-scale.html
I've a MacBook Pro and I've updated the disk to a OCZ Velocity 3.
Pro: faster and cheaper than the Apple SSD disk, insanely fast compared to the original disk
Cons: installation is always a problem (one of the screws that keep the bottom lid closed not do not fit properly and it's a little "outside", I've tried to refit it more and more times but nothing changed), sometimes computer freezes and it seems it could be a problem in the SSD firmware, support of SSD functionalities from OSX Lion needs to be tuned by hand.
So the speed of the SSD it's in the "insanely fast" range described by Jeff Atwood (and every time I boot the notebook I use at work with a regular disk I want to cry because of its slow 7200 rpm disk!) and I will never buy another notebook without SSD disk.
If I have a MacBook to update then I do not know, I'm quite happy with the upgrade (having done it myself) but maybe having the replacement done with Apple parts in a Apple store could be a good idea (if they do it, I don't know).
Consider also that almost all SSD drives on the market are designed and supported mainly on Windows, OSX and Macs are often not considered (i.e.: to update the firmware of my OCZ drive no OSX tool is available, I need to boot using Windows or a Linux live CD).
I upgraded an older MacBook Pro with an OWC SSD a year ago and it was an incredible upgrade:
When I bought a new MacBook Pro I decided to fork over some serious money and buy Apple's SSD as a stock item rather than do the upgrade myself because the price of the SSD from Apple and the one from OWC is about the same.
These MacBook Pros are my only computers and they get a lot of use, in all types of situations and I can honestly say that not only have I had no problems with either SSD (OWC or Apple) but the move from HD to SSD is the best speed boost for a computer you can make. it's incredible.
I routinely buy AppleCare for my computers and if this Apple SSD fails or slows down they'll replace it in a flash (so to speak). I also back up daily with both SuperDuper! and Time Machine so failure of my main drive isn't as much of a worry as it might be for some.
I think there's been a lot written about SSDs that scares people but I've experienced none of it and I won't buy a MacBook Pro without an internal SSD, the quiet, the lack of vibration, the almost immediate boot time, launching and quitting applications very fast and more is incredible.
Solid state is the way to go for at least the system and your main applications if not your entire computer.