I entered using a VW 7 times over the last year, staying for between 1 and 11 weeks on each visit. My girlfriend works there.
There is something wrong on my immigration record, they won't tell me exactly what, but I think they have a record I have previously been refused entry sometime in the mid-90's (I've never had immigration breaches). So each time I turn up in the US they take me in for additional questioning at immigration re: this anyway, but it is a stressful experience....
This most recent trip, the immigration officer really gave a going over. He accused me of working there illegally, and of spending so much time there that I was 'abusing the Visa Waiver scheme', and that I need a visa to enter in future. He said he was going to 'mark my record' so that any other US immigration officer would see that I have been told to get a visa. This will take months, as I was arrested for DWI in the US in the early 90's, so need to go through 'additional processing'.
So answering your question, I'd have thought it unlikely you will have any problems the first few times you visit (thats assuming you're eligible of course). But there will be some point where they insist you get a visa.
I'm considering applying for a B2 visa, but as someone pointed out, staying (long term) with your BF/GF is not a holiday...