The G19's high-profile keys have more directional give than most, meaning they 'tilt' a bit in the direction you push them. However, they're by no means loose, and indeed feedback is quite soft. Despite this you're never in doubt as to when you've pressed a key, thanks to a positive, audible click.
Though they lack a rubberised coating, keys have a relatively soft feel to them that combines with their slightly concave surface to provide excellent grip. In general their slightly narrow top surface compared to many other keyboards (even from Logitech itself) is not a problem, with one exception: due to the large distance between keys I found it more difficult to accurately hit the shift key in the midst of FPS shootouts than on most other keyboards I've used. It's a minor niggle, though, and something you get used to over time.
Build quality of the keyboard itself is excellent, with the tiltable LCD screen attached not to a plastic hinge but a thick metal bar and protected from wear, tear and scratches by a clear plastic shield. The only cause for concern is the detachable wrist-rest. It is held on by relatively flimsy plastic clips, similar to ones which have broken on previous Logitech keyboards I've owned.
As already mentioned there are 12 fully programmable G-keys, arranged in a two-by-two grid and raised higher than the typing keys to avoid mistakes. When combined with the M1 to M3 keys, this gives you a total of 36 different keys to which you can assign keystrokes, macros, shortcuts or even your own scripts using Logitech's G-series Key Profiler. As with the G13, this is easily accomplished thanks to a visual map of the keys where you only need to click on one to assign it a function. You can also record macros on the fly using the MR (Macro Record) key.
However, after using the G13 the G-keys on the G19 come across as a bit of a disappointment, since they're not quite as easy to use or as well laid out. Of course, you do have all the standard keyboard keys, but I can't help wishing Logitech had taken a page from Razer's design manual and made all the keys programmable.
A set of custom profiles for each of your games can be assigned to a limitless number of profiles, and as with the http://www.trustedreviews.com/peripherals/review/2007/11/26/Logitech-G9-Laser-Mouse/p1G9 and G13, the G19 has its own on-board storage so you can take your settings with you to LAN-parties and the like. As with the G13, the G-keys are also very useful for work as well as play, allowing you to automate tedious key combinations and assign profiles to each program you use.