I've been doing more and more long distance rides and I've really been considering going clipless as I've read and people have told me it makes a huge difference. The thing is.. I still occasionally run errands on my bike and switching pedals to do so would be kind of a hassle. Carrying a pair of shoes with me is always a possibility.

Are clipless shoes that uncomfortable? I've been told moutain bike shoes are a bit more comfortable to walk in.. are their any disadvantages to that?

If you go clipless, definitely go with mountain shoe/pedal combo so you can walk in them all day. Touring shoes are very comfortable. That's what I use with my eggbeater pedals.

Clipless does make a huge difference in terms of efficiency and energy transfer. They also make climbing easier. That said I mountain biked with them for years and had them on my fixed bike for a year but for commuting purposes I got really sick of caring an extra pair of shoes or having to walk around in my ugly clipless shoes all day (wearing clipless shoes to the bar for the lose). I switched to clips/straps and I am happier now.

As set if you do go clipless get recesses clip mountain bike shows. I road spds but I have heard good things about eggbeaters. Half plateform/half spd are also available but then you would need to be running a brake to be safe.

If you're not too worried about weight, you could always try one of those combination clipless/platform pedals. Or you could just ride your clipless pedals with street shoes. I had some dual sided mountain SPD pedals on my bike while I was shopping around for platforms/toeclips/straps, and while it felt like standing on bumpy rocks when wearing chuck taylors, it got me places (although i didn't really have much confidence in them)

SPDs are rough if you're (just guessing) going to ride breakless. since they have their locking mechanism on the back of the cleat/platform thingy, it'll cause you to pop out lots when skip stopping and if your SPD cleat is worn just a bit it unclips quite often.

i found that clipless with the style like time atac or eggbeaters work great, the 2 bar clip system. much more stable for skip stops, since their clip system has a solid bar in the back and the front bar moves forward to let you in.

all this is my personal experience though, keep in mind. right now im rocking TIME ATACs and i don't see any reason to change them at all. (thanks 128!)

Yep to all the above. I am using these and love 'em.

I use Cannondale Roam shoes mostly, I also have a pair of Diadora Bike Patrol shoes but I prefer my mountain shoes. I use them on Eggbeaters, it just works(tm).

i got the shimano 324 clipless/ platform pedals.
i only use the platform side w/ clips and straps, but the option is there, just in case.