Depending on what you grill a griddle is a great addition to your arsenal but not the best total replacement for a standard grill. I have a blackstone and love it for burgers, breakfast, etc. However if you ever cook whole chicken, or legs thighs etc, ribs, steak I prefer a regular style grill for that. Currently I have a blackstone and a traeger grill and love the combo.
I would also consider where you would store it. I keep both in my garage. I have years of experience with cast iron cookware so I know about seasoning cleaning this type of cookware. Even with a good cover I can imagine you will have issues keeping a griddle nice outside, unless you live in a very dry environment.
There is a little more maintainence but it's not unbearable. Usually after we eat I got out and heat mine back up, scrape it, wipe it down, oil it, and turn it off. 5 to 10 minutes tops. If I cooked a full breakfast inside it would take longer to clean up three or four pans/skillets then it does the blackstone.