Yeah, the ones I have are plastic too (if you want metal, it looks like Boss has a momentary switch - their stompboxes seem to be fairly well-constructed). Didn't think to check RS:
Sustain Switch
Tape recorder footswitch
1/8" to 1/4" adapter
The second switch looks similar to the first (the picture on the web site anyway), but has the wrong connecter. You still come out ahead $$$-wise if you get the adapter, but then you'll have a fairly long, solid, plug coming out of your SR-16 (which might not be a good thing - easier for stuff to get broken that way).
To answer your question about holding the switch down, no, you don't have to hold it down continuously. There are two switch inputs on the back of the SR-16 - one is for Start/Stop, and has a similar function to the PLAY/STOP buttons. If stopped, push switch to PLAY. If playing, push switch to STOP. The second jack is labeled Count/A/B/Fill and functions a lot like the Fill switch on the SR-16 (in fact, it duplicates this button when playing patterns). You can also use it to tap to set the tempo.
Not sure if you're aware, but you can download the manual for the SR-16 from
Alesis' web site. Scroll down to "Drums" on the right, and you can download three files - the full reference manual, a Getting Started guide (which covers the footswitches on page 9, and also basic pattern and song-writing), and a program chart.
I agree with chris-from-ky - I thought that some of the built-in patterns were interesting to listen to, but there really wasn't anything there that I wanted to use, so I made my own. I'm fairly new to the device, so I haven't done any songs yet - only patterns. The quantize feature is good for keeping you on track.
You can also use the SR-16 as a sound module and control it from your PC using MIDI. I don't think you can program patterns or songs this way, but you can write the songs on your PC using a MIDI-capable program, and then use the SR-16 to generate the drum/percussion sounds.