"This restaurant is very good."
This isn't entirely accurate. It is true that たいへん can have a negative connotation, but it can also be used in this way too, and has been known to pop up in exams to trip people up. Japanese people can use it as shown here, in much the same way we would use the word "terribly" as a positive. "This cake is terribly good". It's not overly common, but it does happen.
It's another word for delicious, but is very informal. Check out this person's explanation of the difference: http://selftaughtjapanese.com/2015/11/16/japanese-word-nuances-%E7%BE%8E%E5%91%B3%E3%81%97%E3%81%84-oishii-vs-%E7%BE%8E%E5%91%B3%E3%81%84-umai-used-to-describe-food/