Reviews by bfields
Odd limitations, can be addictive
It's narrowly focused on making short 4/4 drum loops--making loops of more than a few bars is hard, complicated melodies are tricky given the resolution of the screen (it's small and by default it jams a couple octaves across it). And the individual sounds try to do a lot for you--they have their own hard-wired effects, in some cases they play complete drum patterns for you, etc.

But you can get started experimenting with piling different sounds on top of each other very quickly, and before you know you'll realize you've been at it for an hour.

In the end I haven't come up with anything I thought was that interesting, but clearly it's possible.
Interesting--but not very?
I was curious to play a Theremin, so I'm glad the library has these!

It's a lovely-looking instrument, and the sound is pleasant and clean, but a little boring.

It's fun, at least for a while, to control the sound by waving your hands around in the air.

But after the novelty wore off I lost interest.

You can make warbly swoopy sound effects, but I wasn't able to find much variety there.

You could try to learn to control the pitch and articulation accurately enough to play melodies, and I could see that being a fun challenge, but it would also take a while to really master, and it didn't seem worth it to me for a monophonic instrument without a lot of character to the sound.

But others may find it more inspiring!
The same little ribbon keyboard as the monotrons, but with more bells and whistles. (And higher quality output--I didn't notice the annoying hiss that the monotrons all have.) Includes a simple drum machine and 8-step (16-step for the drums) sequencer.

Fun, and not hard to get started with (though a look at the manual will probably help!).

(Also, note: if you also check out the Bliptronic 5000, and have a couple cables with male miniplug cables on both ends, you can try to sync the two. I could only get it to work well with the monotron set at an extremely low tempo and the Bliptronic at one of its highest. But the results could be kinda fun.)
Line them all up and play them like a xylophone, or hand them out to your friends and family and everyone can play together (thanks to the included 8 mallets). The household toddler enjoyed them immensely. (With supervision!--I doubt they're baby-proof.)
Surprisingly convincing sound, without all the hassle of dealing with actual bagpipes.

Turn the volume all the way up and march around your house playing "Scotland the Brave". It'll make your day.