The Radium 61 is velocity sensitive.
I think you might be getting terminology confused a little bit. Velocity Sensitivity refers to its ability to respond to different levels of force when playing, normally this would result in a louder sound the harder the keys are played although it's possible, as with all MIDI messages, to assign velocity to something different.
Semi-weighted refers to the feel of the keyboard. The Radium has what has become known as "synth-action" keys. Not terribly responsive, kind of cheap feeling, light plastic keys. It's action, IMO is adequate, but not great. The 61ES has a much better feeling to it.
If you want the closest thing to a piano, then you need to get yourself a "hammer action" keyboard. There are a number of controllers out now that boast this feature. The newest and, some think, the best is M-audio's new
Keystation Pro 88, which has a "graded action" this refers to the action getting progressively lighter as you move up the keyboard as it would in a "real" piano. I wasn't overwhelmed with the feel myself, but for the money ($499.00), it's hard to beat.
BTW, your assertion that "the harder you hit the key the louder a tone it will produce...I would figure that it would have the same effect on other instruments...producing a more natural sounding piece", all depends on what sound you are producing. If, for instance, you are playing an organ sound, this behavior is not "natural" sounding at all as organs do not normally respond to velocity. It might not sound natural on other instruments as well.
Also, you should understand that the sliders and knobs are assignable to any parameter you wish. If your software/instrument is capable of responding to it, the Radium can send it! That means you can get very creative! The knobs and sliders are, perhaps, most useful as controllers for soft-synths, allowing you to adjust parameters without having to use the mouse.
Hope this helps!
Ted