Anders,

I've used an MX-900, MX-600, and currently an MX-350.
This may be a mindblower, but of the three, I like the MX-350 the best.
Here's why:
The MX-900 has software issues and poor tactile key feel that results in a lot of "fat finger" errors when pressing keys.
Also, the ultra tiny rectangular keys are scrunched together making this remote very ergonomically unfriendly.
After a few days, I couldn't stand using this remote.
It can do an amazing amount of customizing by computer, but the software has some peculiar bugs.
It has very good backlighting on the LCD and keys, much better than the MX-600 or MX-350.
It's either the software, or the poor tactile key feel that results in errors when pushing buttons.
I suspect the new style keys and making it too complicated are not a good combination.
The MX-600 has a great keyboard layout with great tactile feel to the keys.
In fact, I think the keyboard is identical to the MX-500.
The gemstone "gumdrop" keys are very user friendly, and I can pretty much find the keys by feel in my darkened home theatre room.
The biggest flaw with the MX-600 is the lack of switching between IR and RF.
It transmits both IR and RF, and it cannot be programmed for IR only, or RF only.
If you're using the MRF-100 that comes with it, it causes a lot of double IR signal conflicts, as the MX-600 and the MRF-100 will both send the same infrared signals throughout the room to your devices.
It would drive my Favorites function nuts, causing the channels on my Panasonic Projection TV to have fits.
The MX-600 has 10 keys which means most of your learned or custom commands, or multiple devices, can be all placed on one page, without scrolling.
If you plan to use the MX-600 strictly IR, without the MRF-100, it is awesome for the money, and my pick, as long as you're not doing anything fancy that would require computer programming to do it, like moving functions and pages around.
But if that's the case, you might as well go with the MX-500, as the MRF-100 would be a useless paperweight, which it is anyway, as it has a lot of RF interference issues by nature of its design.
It works flawlessly, with no key input errors like the MX-900.
With the gemstone keys, you know when you've pressed a key, as it gives very positive feedback feel.
I'm currently using the MX-350, and I absolutely love this remote.
Of the three, this is my top pick, with the MX-600 a close second, under the conditions mentioned above (IR only).
You may think going from an MX-900 to an MX-350 sounds like a drastic downgrade, but for my use, it's not.
I've learned the hard way, that simple performs better, most of the time.
Both the MX-350 and MX-600 can control 10 devices.
The MX-600 can do it all on one screen, the MX-350 requires scrolling through two screens.
However, the biggest advantage the MX-350 has over the MX-600 is the ability to program IR and RF control independently.
It can be set for IR and RF, IR only, or RF only.
I love the gemstone keys, and I think it's a big mistake that UR is getting away from it.
It has basically the same great ergonomic user friendly keyboard, and the same reassuring tactile feel of the MX-600.
I use it strictly on RF with an MRF-300, and it blows the MX-900/MRF-300's performance away.
It never has any key press sending errors, and is lightning fast.
The MX-350 and MX-650 have acceptable backlighting, but no where near the clarity of the MX-900.
I consider this relatively minor, as in the end, the keyboard feel and layout is much more important to me than the fancy bling and sophistication of the MX-900 that also seems to bring with it more problems and issues.
The sleek, European, high tech look of the MX-900 grows old quickly as the unfriendly keyboard and keys become a major frustration, and remind you of it every time you use it.
The whimsical, colorful, gumdrop keys may not look as sophisticated, but they grow on you like your favorite computer keyboard.
My acid test is a remote that literally disappears when you use it.
It does everything you want it to do, and you don't even realize it's there when using it.
When using it day after day, month after month, year after year, you want a remote that you never even think about, rather than one you curse every time you use it.