The Clek Foonf is a great seat. I hadn't planned on buying one but my son hated the Britax Marathon (he's also huge and the fit was terrible) and I wanted a seat where he could rear face longer. The Foonf has the highest weight limit on the market for rear facing (50 pounds) and up to 43 inches. A 10 months, my son is already over 33 inches so the high limits were very important to me. I'd rather spend the money now and be able to use this for a few years. The Foonf also has some of the newest safety features, including a crumple zone that is very similar to how a car absorbs an impact.
Overall, it's a really high quality seat, looks great, the install was easy and the fit is fantastic for my son. I also love the Crypton fabric because it's nearly impossible to stain and will never show much wear, if any (we have furniture upholstered with Crypton fabric, too). The cons are, it's a huge, very heavy seat. You will NOT want to move this thing between two cars. Also, rear facing, it takes up a TON of space and if you have a small car, no one will sit comfortably in front of this seat. Like a PP mentioned, it's pretty expensive compared to a lot of other seats but, for us, that wasn't an issue and it really was worth the money. There are definitely a lot of other seats out there that will safely accommodate a large child so even if it isn't in the cards for you to get the Foonf, it's not like you won't have a lot of options.