I usually go business class on both the Starlight and the Surfliner. Access to the LA Metro lounge is a great benefit – I've never had a problem finding a seat, and it's well run.
Business class on the Starlight is a nicer experience than coach, and is well worth the money IMHO. WiFi is a big reason to do it, if you want to get any work done. Motivated business class attendants have been increasingly rare over the past year, but when you get a good one, it really adds to the experience. Your fellow passengers are likely to be quieter and better behaved (there are exceptions) and there are few or no children (again, with exceptions). The bottled water, pillow and voucher are nice perks, although you sometimes have to hunt down the attendant to get them. About 75% of the time, I have two seats to myself, which is really nice.
I like it better than roomette for a day trip. The seats are more comfortable, and you can see out of both sides of the train.
As other posters have noted, there are even more benefits on the Surfliner – a guaranteed seat can be important on some trains, and the snack box and beverages are nice, too (when available – the commissary forgot to load them on my last trip).
Well worth the few extra bucks it costs, on both the Starlight and Surfliner.