I go between square and round pots depending on what application I want to use them for. For the plants I keep in my personal collection, I use round plastic pots that I pick up from Wal-Mart or Home Depot. They're a terracotta color, and they're a little expensive, but I really like the look of them.
For any plants that I make divisions of, or seedlings that I don't plan on keeping, I use square pots. They typically hold more soil than a round pot and I can fit a lot more square pots than round pots in the same amount of space (which is at a premium for me in regard outdoor growing).
I've been getting my square pots (3.5" and 4.5") from a particular nursery on the west coast. Service is top notch and I'm a repeat customer of this place but the pots are shipped priority mail which makes the overall cost a little higher. Still, I love both the design and materials the pots of made out of, so it's been worth it (in addition to supporting a privately owned business as opposed to a major chain).
I'm always on the look out for square pots, but I *HATE* those little flimsy thinwall pots like are used for annuals. I haven't tried any of Andy's pots, but they look like they would do nicely for me.
I'd really like to see a good source for larger square pots <hint>.
I'm like PK... I'd like to see a good meeting of quality and price (don't we all?), but I would definitely pay a premium price for pots (in a variety of sizes) that were designed well, made from decent material, resuable, and preferably manufactured "greenly."
Will