Sadly the 300x600 is becoming very popular and by this time next year I expect it will be mainstream. I run ads via various agencies and also take ads direct, defaulting to Adsense when I don't have "premium" inventory. There is an increasing amount of pressure from the agencies to show the 300x600 now, and it is (generally) paying 25% less than the CPM that I used to get from a standard 300x250 this time last year, rates for which have dropped about 50%.
Personally I think there are too many agencies out there competing to stay alive, driving the prices down in an effort to get the business and dumping the results (and lower rates) on their publishers - but that's another discussion!
They can look good. I'm running one in the UK in my right sidebar at the moment. It has a video playing in the top half of the unit (with sound disabled until requested) and a survey / social section beneath. It really draws your attention.
If you think the 300x600 is bad, look out for the 300x1050 - two agencies have mentioned that they are beginning to sell that size and asked whether I will run it. Again it would show a series of items from the same advertiser - perhaps a video, a survey, some info, a big image - all spaced out within the stack. The rates I have been quoted for this are surprisingly disappointing given the amount of space it would take up on a page.
In my opinion both sizes can work, but only with the web-magazine-style layout you see on major sites (two columns, content in 600px and ads concentrated in 300px sidebar), and preferably image only.
The whole 300x600 filled with text ads as screen-shotted by netmeg is daunting, but again can look OK if carefully placed to one side and if there are no other ads in the same "scroll". If there aren't enough text ads to fill the whole space, or if Adsense starts messing around with the text sizes and bolding (which I am seeing increasingly on the large rectangle unit which I show at the bottom of my page) then it looks terrible and just unprofessional.
Most requests I get these days are for ads that take over my pages. Recent examples have been ads that i) roll down from one corner and obliterate my content completely until clicked away ii) smash down from a leaderboard at the top and obliterate my content until clicked away iii) slide out from the side of the page and obliterate my content until clicked away. You can see that there is a theme here! It really comes down to how much one is prepared to sell one's soul for, in the end. The 300x600 and even the 300x1050 are kind by comparison. These days I love Adsense ads for being unobtrusive, gentle and undemanding!