I get into this a lot. Is Internet Explorer terrible? Truthfully, it's irrelevant — it's the #1 browser.
I get into debates on whether or not it's even the #1 browser. I say yes, according to Wikipedia and a lot of other sources. Some swear by w3schools.com, like my twitter buddy @pissmeoffand, (after all, it does come up #1 on Google. coincidence... probably not), but if you look in the fine print, it states: These statistics are based on W3Schools users.
Wikipedia takes the data from 6 reliable stat services. There is a discrepancy between the two sites, but honestly, I'm more inclined to go with the majority on this one, and all say that IE leads by quite a bit. Some designers are down right shocked that IE is the #1 browser. Why? It's been that way for over 5 years and Windows is still the best selling OS out there, of which IE is packaged.
So, it's probable that w3schools.com stats don't match the others because a lot of developers do in fact hate IE. Again, totally moot.
But, for the sake of conversation, let's assume w3schools.com is right and that IE is the #2 browser.
If you're a front or back end developer, and you don't design for the #2 browser, and yes, I've seen sites simply not work at all, you're alienating almost 40% of users. This isn't good for your client's SEO and their users certainly can't get their information. Of course, if you're targeting designers and you know that most of them are on Firefox, you may be able to get away without optimizing for IE.
Honestly, it's a mistake either way. If you hate IE, get over it. This is business; it's not personal. Design for your client's target audience and be sure your design can be viewed decently across all the major browsers.
Happy designing!