Personally; I'd prefer to recieve plain text email.
The biggest problem you're likely to hit with email + CSS is linking... for the same reason pictures from remote locations don't load by default; linked CSS ( i.e. link href="" ) isn't going to load by default. There's a risk of someone operating spam emails that invoke cross site scripting attacks (webmail only) even attempt distributed denial of service attacks ( webmail or email applications ) if remote resources are requested upon opening an email.
The only other problem you might run into would be if email clients used their own stripped down parser/renderers; and if those renderers didn't implement CSS fully. For the reason I've already mentioned; CSS
shouldn't be implemented fully in email clients; because there are CSS properties that can invoke downloads from remote locations ( background-image for example ).
Ideally though; an email client shouldn't block all CSS to prevent such occurances; it should block whatever module it is that fulfils remote downloads.
How exactly are you using CSS?
<span style="font-family:Arial;">inline?</span>
<style type="text/css">
span{font-family:Arial;}
</style>
<span>embedded?</span>
<link rel="stylesheet" type="text/css" href="http://yourdomain.dot/email.css"/>
<span>remote?</span>
The only other problem you might hit is rendering differences in email applications, if they use OS specific controls; those'll likely be the system's lowest common denominator (i.e. My old MS Outlook will use the same HTML rendering component as MSIE5; regardless of the fact Opera is my default browser ).
These pages might help; they are more related to HTML email in webmail clients. Still; CSS is more likely to be actively stripped out of webmail views as apposed to just being ignored, so perhaps they'll help you somewhat..
http://alistapart.com/articles/cssemail
http://www.campaignmonitor.com/blog/...ing_css_1.html