It's actually not too bad, even on phones without "easily" replaceable batteries.
Speaking for the OnePlus One and Nexus 5 the procedure is pretty straight forward: pop it open using plastic trim tools (which are often sold with batteries) disconnect the ribbon cable and pry the battery off any adhesive. Drop the new one in, reconnect it and snap it back together and you're done.
Depends on the phone I guess, but the whole thing takes maybe 15 minutes.