Two years ago, I've switched to a coil spring shock on my Ibis Mojo HD and never looked back. I always wanted to upgrade it to a Titanium spring, but it was kind of difficult to find one compatible with RS Vidid at the required length. Recently I've changed my shock to Fox RC4 and found one spring available in an online auction.
There is much debate going on about benefits of using Titanium spring on mountain bikes: weight reduction, less unsprung weight, better small bump sensitivity (really?).. If nothing else, it just looks cool:)
As for weight savings, stock Fox 2.8"x400lbs spring weighs 404g. The 2.8"x400lbs RCS Titanium spring weighs 294g, resulting in a 110g (27%) saving. RCS Titanium spring is also little bit longer (maybe 3mm), but there is plenty of available space on a Fox RC4 shock. Spring lengths can be little bit confusing, though. Some manufacturers claim the length of a spring in the shock stroke length (2.5" spring for a 8.5"x2.5" shock). Others (like Fox) claim the free length of a spring (all the way it goes before it's completely compressed). So even when Ibis Mojo HD uses a 8.5"x2.5" Fox shock, you need a 2.8" long spring.
The spring is already mounted on the bike and some picture will follow soon;)
Very few things can be learned just from reading a book, but this one has opened my eyes and I realized the mistakes I was doing. Well written with great images it's an investment well worth the money. Check it out