Miche has the advantage of being completely customizable for tooth counts. To do that, they use full, loose cogs (no spiders), so the weight will be more than it would be for a weight-optimized cassette with spider and the inner portion of the cogs cut away. I suspect the weight difference is about the same as one or two extra energy bars in your back pocket ... not worth the worry, IF you can get the gears you want.

I've successfully inserted individual Miche cogs into Shimano cassettes on my half-bike(s) in order to get certain sprocket progressions. As mentioned above, Universal Cycles (and others) sell the cogs separately.

Miche also makes both Shimano and Campagnolo-compatible cassettes (you can't mix between them, but they make cogs that will work on the two).

There is a vendor on eBay UK who will make up custom cassettes using Miche parts. Cost, with shipping to US, is about 45 UK pounds. This is a less expensive approach if you want a whole cassette and not just a cog or two.

It's worth it to spend some time with one of the on-line gear tools ( http://home.earthlink.net/~mike.sherman/shift.html by Mike Sherman is a nice one; has many of the commercially-available ones already there to try out) and really think about what range you need. The "semi-log" graphic helps point out the flow and the relative jumps between gears.

On a single, for us non-racers, about about 30 or 35 mph downhill it is faster to tuck than to pedal on a single bike, so a gear above 100 inches is wasted. That means that if your big chainring is more than 48 teeth, those 11 and 12 tooth cogs are just taking up a space in the cassette that you could use somewhere between say 16 and 20, and which the cassette maker might have decided that you don't need. High gear choices for a tandem migh vary, but it is still nice if the cyclist, and not the cassette manufacturer, can pick what they want. Like in the old days of 5-speed freewheels and the board of cogs in the bike shop.