At best, the reaction is going to be shrug-who-cares or, for the truly oblivious, "I didn't notice". More often, it's going to range from irritation through "this honks me off so badly I'm gonna make a blog entry" to "no way am I going to buy anything from these annoying fuckwits."
It's almost as bad as a pop-under ad. The usual phrasing of "going to a website" isn't exactly the right metaphor -- you're really inviting the website onto your machine. And just because I invite you into my home doesn't mean you can rearrange the furniture without asking.
If you do insist on the "going to" frame, then think of "going to" a store, and as soon as you walk in the door, someone grabs you from behind and ties a tie around your neck.
Edit: Company websites shouldn't just be ads, either. They should have useful information, vital statistics for products, maybe even PDFs of user manuals.