This pattern conveys that the degree of something is beyond description, often used for strong emotions or striking impressions. It is similar to saying 'there is no words for how ...' but in a hyperbolic, emphatic way. It is typically used in exclamatory statements, and while it is not rude, it is quite dramatic and suits casual or emphatic speech, less so for polite conversation or writing.

For verbs, use the dictionary form (e.g., 驚くといったらない). For な-adjectives and nouns, attach directly to the stem or noun (e.g., 不便といったらない). For い-adjectives, attach the dictionary form (e.g., 嬉しいといったらない). Note that the pattern often appears after a noun phrase, and the whole phrase can be followed by は or が, as in その光景は美しいといったらなかった (past form).

It is a set phrase, and for past situations, use といったらなかった instead of changing the ending.