I wanted to review this since I agonized over the purchase for 2 months. I've been using an adapted 100-400 zoom with a 1.4x extender. Though I get great images with that combination, there are still subjects that are too far away. I was debating on a 2x extender or this lens. Both have been back-ordered and this lens was back in stock first so I took a chance.Before I get into the pros and cons, I will start by saying that I would recommend this lens as long as you understand what you are getting yourself into. It isn't as well built nor does it produce the image quality of an L lens but it costs 1/3 of what my 100-400 cost. It is also infinitely less expensive than a prime L.Pros:-Reasonably fast focusing. I was able to attempt some birds in flight photos. I was able to track and take pictures but there was not enough light for higher shutter speeds that are necessary for clear images.-Lighter than an L lens since there is not as much glass nor is there much metal. The only noticeable metal is the mounting ring.-800mm for an incredibly reasonable cost. I was able to take some image that I would not have even attempted with my 100-400 w/1.4x costing much more.-Decent image quality. My first outing was an overcast morning and I was hand-holding. Though I got some reasonable images, there was some obvious camera shake. Based on the Grebe chick image I included that was taken with some sunlight, hand held shots are possible. A monopod is recommended.Cons:-Center focus area. This isn't an issue for distant subjects. It does become an issue if the subject is a little closer. I am used to the edge-to-edge focusing on my R5. There were some situations where I could not get eye focus if I wanted to frame my subject properly. I could have swapped out for my other lens but that is at the risk of missing a shot.-f/11 aperture. Having stated that, it isn't really an issue in my case nor would it be an issue if you own an R6 as well. You can really push the ISO on the R5 and R6 before noise becomes a problem. The Cinnamon Teal was at 2500 ISO. Anything below 8000 is fine on my R5. On this overcast day, the highest ISO reached was 6400 and that was because my shutter speed was at 1250 for 'action' shots. Lowest ISO was 800 at 1/320 shutter speed. I could have gone slower but would have likely encountered worse camera shake.-Not as well built as an L. It doesn't seem 'cheap' as I have seen it described but the jury is out on how long this lens will last. More important, it is not weather proofed. I can also see potential issues with the slide mechanism used to put the lens in shooting mode. Sand or dirt could cause problems.-Lens hood and tripod ring/mount not included. Though cheaper than an L mount lens that will come with a both, Canon could have at least included a hood.Again, I am happy with my purchase and would recommend for those that own an R5 or R6. The reach is incredible for the price. Having spent a morning with the lens, I have a better understanding on how to use it. As for whether a 2x extender is a better option, it could be if you already own a good 400mm lens. You would still experience aperture and center focus issues like you do with the RF 800mm. There are also some EF lenses not compatible with the 2x.ADDENDUM: I was able to take the lens out on a sunny day. The higher shutter speed on bright days makes it easier to hand hold the lens. For reference, the Killdeer image is taken at 1/1000 with an ISO of 1000 due to the dark background. The eagle 30 feet up in a tree is cropped taken at 1/1000 and 1600 ISO. Though not tack sharp like an L, it is still pretty good. Subjects with a light background will drop the ISO with the lowest I saw was at 500 with a 1/640 shutter speed.