smugmug is great if you want to show the highest quality online version of your images and, IMO, is the place for pros and enthusiasts who intend to sell or earn from their photos; the order/print tools in place are outstanding. It's also a much higher caliber community than flickr, generally speaking.
Flickr is great for hosting/sharing, for the API features (particularly greasemonkey scripts with Firefox) and the social community ... if you like that aspect. There are a LOT of nutjobs, uber amateurs, etc and the countless 'pools' (groups for everything you can imagine) mean invites are inevitable. I like the social aspect, for the most part, but it can virtuous (lots of circle jerks on flickr).
I also think flickr suffers from general trends -- cliche photos, if you will. HDR, vivid seascapes, nekked women, flowers, etc will always get you lots of views (and with it said group invites and nutjob followers). Flickr is also insanely narcissistic; I can't believe how many people are so full of themselves (and how many sheep follow them).
And the one thing that bugs me most about flickr, but can be a benefit, is that they screw with images after upload (they clearly compress, despite saying they don't). I like that this means someone trying to pirate my stuff isn't getting a great copy ... and I rarely upload anything bigger than 800px at 72dpi anyway. But it also means my stuff isn't as clean and sharp on flickr as it really is. You almost have to post process with flickr's inherent compression in mind to optimize how your photos will look once uploaded to your stream.
I know a lot of people who use both flickr and smugmug (myself included). Flickr is hard to beat for $25/year and unlimited bandwidth, especially if you're using API tools (either for viewing, like scripts, or via your own web presence/interface and flickr as a backend).