I'm so surprised tabs is so popular; I always thought 4 spaces is the common standard (in a general, overall, way). I use 4 spaces.
For complete clarity, I'll just point out that the tab key is not related to how you indent (tabs vs. spaces). We can/would use the tab key in either case.
Anyway, the thing with tabs is that it's not shown the same for everyone, and it's basically the only reason I choose spaces over tabs, if I can. As a top of my head example, for code written like this:
someFunctionSomewhere(passing,
all,
sorts,
of,
things)
Tabs can potentially be annoying here. Person that wrote it had to use let's say, 6 extra tabs to format like that (because for that person a tab is about 4 spaces), but when someone else tries to read it, and they have a different setting for tabs (maybe they prefer 8 spaces for a tab, or 2), suddenly it shows up weird, might even need to scroll to read it.
So yeah, that's the only thing about it for me. Spaces basically fix that kind of issue. However, it doesn't beat established guidelines and such, as others pointed out. (Though it might be a valid reason to start a discussion about adopting a spaces style. :) )