These are all fully functional so you can test-drive every single feature that's available in the retail version of the software. In terms of new features, Adobe claims more than 250 have been added throughout the Master Collection and several of its applications have been updated to take advantage of things like multi-core processors and GPUs. Photoshop, Premiere Pro, and After Effects are now 64-bit native, both on Mac OS X and Windows, and particularly improvements in Photoshop's OpenGL engine will make the new version faster and more responsive as well.
In total there are new versions of fourteen point products (all except Acrobat Pro), and a brand new software called Flash Catalyst which is included in every edition except Design Standard. Adobe has also included a set of five online services that complement CS5 products and are said to augment key aspects of the creative workflow.
At around $2600 the complete suite doesn't really make much sense, but Adobe's targeted editions -- Design Premium, Web Premium, Production Premium -- might be right up your alley if you rely heavily on these tools, or perhaps even getting them individually. CS5 is definitely not for everyone, but that shouldn't necessarily stop you from trying out the free versions (after registering on Adobe's site). The Illustrator trial weights about 1.3GB, Photoshop over 950MB, while Flash is closer to 850MB. The Design Premium and Master Collection are 2.8GB and 4.5GB, respectively.