The new MS keyboard may be nice, but It would never replace a g-15 for me.
Actually, I'm on my third g-15.
The First got replaced due to the Key Tops wearing away. It was a common problem with the first released keyboards and I got it the week it was released. Fortunately, Logitech replaced it free of charge.
The Second was also an older version. Unfortunately, It didn't like coffee as much as I did. It didn't suffer from the same key top problems that the first did though.
The Third, and Current, g-15 I bought to replace the bad keyboard. This time it was the amber one. Overall, I like it a bit better than the "blue" one.
The original G-15 has better media controls, I like the adjustable screen and the many more programmable keys were nice. But, realistically, I never use any of the macro keys. I keep telling myself I will, but I haven't yet. Nor do I ever adjust the angle of the LCD screen. I opened it up full and kept it there.
In the end I liked the "amber" g-15 because of it's smaller foot print, the LCD is easier to read, and the Amber color keys is much better than light blue.
I'm with Scheibler on the usefulness of the LCD screen too. A lot of people may find it gimmicky, but I can't live without it right now.
I use it while playing EQ2. When on a Raid with 24 voices I find that using Ventrilo's G-15 support to be very useful since I can see who is talking. The DPS parse application I use also supports the keyboard so I can keep track of my damage at a glance.
There are a lot of games out there that now support the g-15 LCD panel, though those two applications are always the ones I use it for.