I've had my long arm for a while now and I adore it - I thought I'd use rulers all the time but truth be told - I don't! I have a stitch in the ditch ruler that I rarely use - after a while ditching straight lines is pretty easy by hand unless it's really long or awkward. I personally find that I only really use rulers at this point in my quilting for circles or diagonal lines.
That being said if I were to invest in rulers I really really like Rhonda Beyers rulers from Quilter's Rule.
She's got one for just about everything.