Autofocus was overall accurate and reliable. I wouldn't necessarily call it slow, but its also by no means the fastest lens I've used. It was perfectly suited for my mostly stationary senior portraits, and I imagine it would also be perfectly capable of a bride walking down the aisle. As someone who has shot a lot of indoor sports in the past, I don't think this would be my first choice If I absolutely needed to capture a sharp image of fast action, but the gorgeous image quality certainly makes up for a few potentially missed shots in my opinion.
Of the 5 portrait sessions I shot in the course of my first weekend with the lens, its worth noting that 2-3 images from each session were noticeably back-focused. I'm not certain that I can blame the lens on these missed shots though, as a 1.8 aperture at 135 results in a razor thin depth of field. You'll certainly need to be a little more aware of your shooting technique when shooting with this lens as compared to something like a 70-200 F/4.
Sigma should be applauded for their innovative and unique lens lineup which they've very thoutfully developed over the last several years. The 135 1.8 Art is absolutely a worthy addition to the Art Lens lineup, and provides something that none of the first party camera and lens manufacturers offer photographers.
While its size and incredibly shallow focus plane may be a little intimidating to some, portrait photographers will be rewarded with stunning images if they take the time to master it.
At $1399 retail, I think this lens is also a very nice value. It sits right in between most companies 70-200 F/4 and 2.8 in terms of price, and the performance is arguably better. Wedding and Portrait photographers looking to add a unique tool with equally unique (in the best way) images to their repertoire should most definitely consider giving this lens a rental or purchase.