A great client is Shaun Inman's Fever[1]. It scales really well; the more feeds you add to it, the more tailored it becomes to your interests. It's not free but it's easily the best aggregator I've seen.
Nice to hear, I am not the only one who uses RSS for extremely intense reading of all web sites and blogs I admire. Average users are heading more like to a TV-like reading mode; which is close to "passive". But It is nice to see there are a lot of hackers who want to "actively" select what they want to read.
[1] http://feedafever.com/