Personally I wouldn't worry too much about IS, a good tripod will do the job just as well... that said I've never used an IS lens so perhaps I don't know what I'm missing. If you do go for an IS lens have a look at the Sigma 80-400 OS (OS is sigma's version of IS) as well as the Canon 100-400 IS.
If you can afford to go for a prime lens and are happy to carry the extra weight, I am sure you won't regret it. I changed from a Sigma 170-500 zoom to a Sigma 500 f4.5 prime last year and am very pleased that I did. True the zoom was about 1/3 the weight of the prime and could easily be used handheld, but the prime is so much sharper and faster. I haven't missed the flexability of a zoom at all, for birds I used it at 500mm 95% of the time anyway.
But really it all comes down to budget and weight - my budget was limited, but I don't mind carrying the weight so I went for a secondhand Sigma prime. If budget hadn't been an issue I'd have got a Canon 500 f4 IS. If weight had been an issue I'd have stuck with the 170-500 zoom which is a very good lens for the money.
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