This hasn't come out of my PSP since I bought it a month ago.
Beaterator is very fully-featured for a console music making program, especially a handheld one. It strikes a good balance between keeping it simple enough for novices who just want to drag pre-constructed loops around and get something listenable, whilst offering enough depth that people used to 'proper' music making software/hardware won't find it frustratingly limited.
There are lots of good pre-built loops (both by Timbaland and Rockstar's very capable in-house sound guys) with decent 90s DnB-style time stretching on the loop player. You can build your own drum patterns up in a good Roland x0x-style sample based drum machine too. It also has a basic 3-oscillator semi-modular synth included. There's a piano roll for melodies and 2 fx slots for each of the 8 channels. I've just started importing one hit samples and loops into it from .wavs on a PC and it works really well. Nice Recycle-style automatic beat slicing capabilities too.
I also find the very quick boot up time of a hibernating PSP means that it is ideal to keep by the bed so if you wake up in the middle of the night with an idea and need to get something down quickly, it's really useful.
It's obviously never going to replace proper studio gear/a dedicated music laptop, but for quick sessions on the train/at airports/lunchtime it's very good. The fact you can export finished tracks to .wav on the PSP's memory card, so you can get them off onto a PC for burning to CD, MP3ing, uploading, etc. is good as well. Makes it handy for 'sketching' tracks on the move semi-pros, then polishing/mastering them when once back in the studio.