Just finished scanning over a comments thread on another site, where Buddhism came up.
I admit, I've always had trouble taking Buddhism too seriously. Partly because it seems that Buddhism doesn't take itself too seriously, and the Buddhists I've run into don't take Buddhism seriously.
There's a few aspects to that. First, Buddhists in general seem allergic to definitive statements or rational discourse, to the point where their whole religion seems very fideistic. I've run into very little in the way of defending or even explaining buddhism philosophically, etc, certainly in a definitive manner. Oh sure, there are some core principles - the four noble truths, the eightfold path. But beyond that?
For instance, I've heard - repeatedly - that Buddhism is an atheistic religion or philosophy. That's far from apparent, even after setting aside all the small-g god / Buddha / boddhisatvas. Apparently Buddha had a bone to pick with Hinduism, but didn't lay down a teaching about God's existence or non-existence, regarding the question as a distraction. Belief in some kind of theistic ultimate ground of being is not only possible, but it shows up in some Buddhist schools and Buddhist thought.
But hey, that's besides the point, right? Buddha just regarded those questions as distractions. Buddhism focuses on important things, the matter at hand - enlightenment, extinguishing the self! But then you're going to find that there's wide disagreement over what this damn "self" or "no-self" is. Which I suppose is fine, since you can also expect to be told "There is no self, and there is no no-self." As usual, the Buddha didn't seem too concerned with making this question clear, and again that helps to explain the various traditions. It isn't like this is a minor question either, since the issue of self is central to the entire philosophy and approach.
And this is all just as well, since the earliest Buddhist scriptures are dated to roundabout the first century AD. And when did Buddha live? Roundabout 600 to 400 BC. So take all the various hyper-skeptical views you see people subject Christianity to and multiply it by ten or so. And it's not like you can say "Well, Buddha's word for word teachings aren't important. Buddhists have experienced what he taught for themselves!", because there's disagreements between various Buddhists over some damn essential issues.
I'm not mounting anything approaching a serious argument against Buddhism here, and I'm being very damn cursory as I give my impressions of what I've read about it. And I'll say right away that Buddhism, in terms of moral teachings, has quite a lot in common with Christianity. There's a lot of common ground there. But it seems like, more than any other major religion, Buddhism does not even attempt to provide a rational argument for or defense of itself. It's almost purely a matter of accepting it at face value and committing without argument to one of a number of views that fall under its banner.