I had been quilting just two or three years when I purchased my 26" Innova on a 12' frame. Like you, I wanted to finish my quilts myself. I've been entirely happy with the purchase. Before I got the longarm I quilted my larger quilts in sections on my DSM. I hated the layering process, and even though I was quilting in sections my shoulders and back were sore after quilting for any length of time. I convinced myself that I would recoup the money invested in the longarm by not sending the quilts out to be quilted by someone else. I think that has been true, but it's probably not a good reason for buying a longarm.
All of the people I've known who purchased a true longarm (18" or larger throat) made by a major company (e.g. Innova, APQS, HandiQuilter) have been happy with their purchase, whether their machine sits on a frame or is a sit down model. From what I've observed, those who start out with machines with smaller throats or lesser quality frames usually get frustrated and upgrade at some point.