Formatted signatures in Outlook have been a persistent problem – here’s my writeup about workarounds to make signatures arrive correctly using Outlook 2003.
Here’s a description of another reason to be cautious using signatures with heavy formatting or graphics. The author discovered that his outgoing messages were far more likely to be marked as spam when they included a logo in the signature.
I have no way to evaluate whether that’s a widespread problem. The author’s experiments were limited to Outlook 2007 and the spam filter in Microsoft Exchange Server, and I haven’t seen this mentioned anywhere else. It’s a serious problem if you send an e-mail message and you can’t be sure if it makes it to the recipient; if using a simple Outlook signature increases those odds even a little bit, it seems a small price to pay.