Hi there!

Unless I've completely missed it, this appears to be a new feature. How does it differ from adding a word?

Also, would it be a good idea to add all expressions (sentences) and words I hear in my day to day life haphazardly into the Miscellaneous list and have it add the default date when the entry was made, or should I perhaps make sections of my own?

/Shanghainese

Hard to say. I have added lots of sentences since the feature has become available, but it eats up lots of time when studying. You do learn lots of useful sentences and revise word order and changing tones so I think it's worth it.

For all intensive purposes, the only way it differs is that it's a longer chunk of information. It's your call as far as how you'd like to study. I'm thinking it might be useful to have separate categories for sentences you're exposed to, such as literature, newspaper, slang, academic, etc; as the style of composition largely depends on the context in which the sentence is expressed.

I haven't used this feature yet so I was wondering if it would useful for phrases such as 談上戀愛了(tánshàngliàn'ài le) / to have started dating".

There is a fundamental difference to a word and a sentence. A sentence, as the name says, is an example sentence for the usage of a word.
You have to pay admost attention to the sentence formatting. To take jfcaca's example, you have different options of formatting:
1. 談上戀愛了, (all characters together), this would be the same as a word, because it will never pop up for any word.
2. 談上 戀愛 了, this would be an example for the 3 parts 談上, 戀愛 and 了 and it would pop up as an example for each of the 3 words / character, provided, you have selected it for the 3 words also (see below).
3. 談 上 戀 愛 了, all characters separated, then it would pop up, when one of the characters is reviewed, however, it will not pop up, when the words 談上 and 戀愛 are coming up for review.
What I do, once I have entered and formatted the sentence is, that I put them into a list, click on the word, where I want to have it as an example and search through all the example sentences, find it and select it as an example for me. This may be quite cumbersome, because if this sentence is selected for the first or second time, you might have to go through quite a big number of example sentences, before you find your own.
Other thing, which you should keep in mind is, that the sentences are public. So everybody might see it and might edit it, if someone finds the info, which you put there, makes no sense. @shanghainese, so somebody may think, what a strange date there and removes it. Same may apply to the formatting.
Also pay attention to the pinyin. Sometimes, the formatter has difficulties with the de5 and the le-liao, etc.

Is there also a difference between a set phrase (e.g. 不好意思 or any 成语) that consists of words and/or characters but may not be fully understood unless specifically learnt, whereas a sentence should be understood if all words in it are known. What would you add phrases as?

What are the consequences of adding as sentence though (can't see in faq - was this on a newsletter) - possibles I can think of are (a) not shown in progress totals, (b) changes parts studied (no writing?), (c) as Roland indicates is used in example sentences section for words.

Regarding (c), I guess there is the question whether 不好意思 is an example use of 意思 or too divorced from the word meaning to count.

不好意思 is of course a relevant example use of 意思. The following is a dialog showing the different uses of the word 意思.

局长：“你这是什么意思？” （意思是为什么要送红包）
小李：“没什么，意思意思。” （心意）
局长：“你这就不够意思了。” （没有必要）
小李：“小意思，小意思。” （小小心意）
局长：“你这人真有意思。” （明白事理）
小李：“其实也没有别的意思。” （特别的用意）
局长：“那我就不好意思了。” （收下了）
小李：“是我不好意思。” （红包不多）