I've used Michelin, Pirelli and Techno mousses in various bikes.
My Michelin rear mousse got very soft after ~12 months and 2,000~2,500km. It had a hard life - the 24 hour, the A4DE and a 12 hour, plus a fair bit of trail-riding in there. It was a big sized one (Michelin 140/80, IIRC), but I'm pretty sure it would still work fine in a 100/100 tyre (Michelin 120/90).
The matching front is now near the end of its life (18 months, probably 2,800km) - its still OK for trail riding, but I wouldn't race on it now and I reckon I'd be far more comfortable using a rimlock with it.
Michelin says that they last six months (virtually regardless of useage), so I don't feel ripped off.
The Michelin ones start off a bit harsh, but feel like a tube at ~13psi for most of their life. It's a different feel to a tube - more of a 'dead' feel to it, but I actually prefer the way mousses feel. Some people talk about revalving their suspension to suit mousses, but 99.9% of us don't have our suspension even close to needing that sort of detail...
The Pirelli is about the same as the Michelin - I can feel far more difference in different tyres than I can between the two brand-name mousses.
Haven't had the Pirelli for long enough to comment on longevity, but they're supposed to be slightly superior to the Michelins.
The Technomousse supposedly doesn't have a defined shelf-life - they will eventually wear out with use, but not with time. They feel very 'dead', both in your hand and on the bike, but you quickly get a feel for them. Definitely a lower performance option than the Michelin/Pirelli mousses, but they're still quite good and about half the price.
A mate runs them (Michelins mostly), and is religious about lubing them - to the point that he will remove a good tyre just to re-lube the mousse. He gets exceptional life out of his mousses, so there's probably something in his theory.
That said, he and his missus had a Michelin cause them to DNF this year's 24-hour (on the back of their sidecar, so it was really punished). Several sidecar teams DNFd from the exact same thing this year. Seems that the bike shop that fitted them either forgot to lube them, or used very old stock mousses and they just didn't last.
He's back to fitting his own...