I have them installed on my daily driver too. I also have the 9005 conversion on the Z, but the bulbs last at least 2.5 yrs on it, since I change them anyway (whether burnt out or not) because any bulbs get crappy after awhile, you prolly already know this but, Silverstars are limited in lifespan too. That is according to Sylvania: [ http://www.sylvania.com/ConsumerProducts/AutomotiveLighting/Products/ProductComparison/default.htm ]
Although my buddy in a '99 Saab 9-3, did the Silverstars and had one bulb burn out after a few weeks, but he was able to return it & got another one. But no more probs since...