They have good stuff, and they generally guarantee stuff to work and provide a warranty on much of the stuff. Their service is fast and friendly.
That said, their prices are way more expensive than most places for many of their items. But some of the prices are reasonable, and the guarantee is nice after dealing with lots of stupid "it probably works but sold as is" ebay idiots who are selling items they know don't work.
I would recommend oldsoftware.com but ONLY if you can't find what you want somewhere else first, or if you absolutely can't live without a product guarantee.