I have a 22.5 OTG and love it. It's great for steaks - the size gives you more than enough room to go indirect if that's your thing. I think it is totally worth the price.
Ironically enough, one of the pros is also one of the cons: the size. As I mentioned, it gives you plenty of space, but you have to heat that space, meaning you will likely use a bit more charcoal than in an 18.5" OTG. For me, the pro of having more space outweighed the con of more charcoal usage.
Another pro (to me) is the ash catcher system. I also have an 18.5" OTS and I like the ash catcher in the OTG much better than the pan on the OTS. I have a dedicated bucket for ashes from all four of my charcoal grills (I fill the bucket, let the ashes cool, then dump them in the trash), and it's much easier dumping from the OTG than the OTS.
Beyond that, there really aren't any cons that I've come across. It's a Weber, and that means quality construction and a well-thought-out, throughly tested design. If I were to do it all over again, I would skip the 18.5 OTS and go straight to the 22.5 OTG because I think it is worth the $150 price tag.
EDIT: Forgot to mention that it's only my wife and I around here. There is typically alot of wasted space on my 22.5, but I usually just grill the meat. I don't grill veggies or potatoes like some people here do. If you grill things besides meat for your meals, you'll be happy you got the bigger grill. If you're like me and just grill the meat, expect some wasted grill space if you're just grilling one meal for the three of you.
