I built my first sidemount rig in spring 1999. I had just come back from an expedition in Merida Mexico and decided I wanted to focus on some of the more "off the beaten path" places in the area. I had already been cave diving for 5 years and had probably 500 or so cave dives under my belt at that time.
The only commercial sidemount rig that was available back then wasn't that great, so I followed my friend Derek Hagler's lead and assembled my own sidemount rig out of 2" webbing, bicycle innertube, and a Seatec wing. What we assembled was very similar to the modern Razor system, but it had a bit more lift because we were using a seatec wing, and aside from the wing, it cost me all of about $50 in parts and an hour or two of my life to assemble. I dove this rig almost exclusively between March 1999 and May 2001 when the death of a friend caused me to give up cave diving for awhile.
For me, sidemount opened up access to a number of caves that we could only get to by boat, such as Rock Bluff, and access to caves that needed sidemount to get into them (such as Rock Bluff). Trying to get a set of double 104's in and out of a canoe was a nightmare.
Now, when I feel the need to dive sidemount, I use a Nomad LTZ but I'm supposed to be getting my hands on a ScubaForce Blade sometime soon.