Upgrading from a factory Mac drive to something like the WD Cavier Black will yield many gains, including both speed and reliability. You can get the 750GB high performance drives (WD Blue/Black, Momentus, etc) for around $125-150 and the 500GB for $100-125. People who buy these drives are very satisfied with them and agree the price was worth the speed gain.
Upgrading to a SSD will take data movement speeds as much as 50 times faster than with the standard hard disk drive (HDD). However, that does not mean your computer is 50 times faster, but it does mean quick boot times (I am about 15 seconds) and insanely fast load speeds. Also, FireWire 800 and especially USB3 transfers will be able to show their true speed only with a SSD. SSDs are essentially better in every way that magnetic drives except the fact that they are much more expensive and have a max capacity which now sits around 600GB. The 512-600GB SSDs are so expensive that I doubt any average consumer would buy them. Most people on this board use the 64, 80, 100, 120, 160, and 256 gig sizes with 120 seeming to be the most popular, and it seems to be the best blend of size versus costs (I have one myself). I would argue they are worth their weight in gold and worth buying now as you get to enjoy (among the many benefits) the insanely fast speeds and you have great reliability and durability, including impact resistance which HDDs are very sensitive to. Flash based memory is just an amazing thing all over and it is getting more and more popular each year.
Quality wise, the current SSDs are very, very durable. The cited standard of SSDs is often the Intel X-25. If you look at the reviews of the X-25, you will see hundreds and hundreds of amazing reviews and very few people regret purchasing it. As of now, there are numerous SSDs on the market which perform very well and have the framework for long-term durability. Also, the lack of OSX support for SSDs has been mitigated by controllers (SandForce 1200) have their own wear-leveling software.
If you upgrade the HDD, you'll be happy. If you get a SSD, you will be in shock with how fast programs run. IMO it is currently worth the money but whether it is worth the money is a completely individual call.