The Database has 2,563,945 games - all the way up to Sept 2011, which is cool.
I know what you mean about it constantly wanting the disk installed - I haven't noticed this, but have actually had the DVD in there the whole time, so it may already know that it is there. This was an annoying feature, and hopefully they got rid of this.
If you use the "Full Analysis" and "Blunder Check" features, you are not going to notice any differences in the analysis. I have noticed over the years they got rid of some of the stupid language output - for example it used to be if you played pawn moves like h3, it would always say "controls g4", regardless if this was important or not.
The "Let's Check" totally changes the way that analysis is done. So especially with openings, you can see what types of analysis others have added, and with all sorts of different engines which is very useful, and interesting. The "Live Book" is very cool, and used in conjunction with the regular book I think it very powerful.
Basically you can see what the traditional theory says about a position, what the database says about a position (percentage of games won/lost/etc), and then also what various computer engines say about it.
The computer recommendations, and the "book lines" are often quite different as well. For example in the Kings Gambit, the computer engines love an early Nc3, but the in book theory, this move is often played much later.
Another feature I didn't mention in the post is the "Monte Carlo" feature. Basically you can pick a position that might not have a lot of theory behind it (for example, the start of the Fishing Pole 1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 4. 0-0 Ng4!?) then let the computer play thousands of games against itself from this position, and create it's own opening book. Pretty cool.
So the TL;DR (too long didn't read) summary is:
"If you care about the Lets Check feature, it is worth an upgrade, if you don't, then don't bother", which I think others have said as well on this forum.