So Lost season 5 has been awesome. Best season of the show so far, with the plot as fast paced as it's ever been, excellent characterization and character development (especially in the last two episodes, The Life of Jeremy Bentham and Lafleur) and some damn cool sci fi ideas. Good job, Lost.
I watched Buffy season 4, which unfortunately was pretty disappointing. There were some pretty amazing individual episodes (Hush, Fear Itself, Restless the episodes with Faith, Wild at Heart and Something Blue) but the main plot line about the Initiative was seriously awful; badly written, plotted, acted and everything, even if it seemed like a cool idea at first. And Adam is pretty much the most boring villain ever.
And now I'm watching Angel season 1. There's some good potential here, but so far there have only been monster of the week episodes with few hints of story arcs- hopefully something will get started soon.