My understanding of using わ at the end of a sentence is that it's essentially just for emphasis, just like using よ, and that only women can use it.
However, as far as I know, women can, and often do, use よ as well.
So is there a difference in meaning, nuance, or context which would make a woman choose to use わ instead of よ?
よ expresses that the speaker assumes that the recipient does not know what is said.わ does not have such implication, and it just adds feminine flavour to the sentence.Disclaimer: I'm just a random Japanese native and my answer below isn't based on formal research or anything like that.
The feminine 「わ」 seems to have become almost extinct. You see it in text books and novels, but it's extremely rare to hear people actually using it.
The kansai 「わ」 is different from the feminine 「わ」. The feminine 「わ」 is used in 標準語 or 東京弁 and not in 関西弁. The kansai 「わ」 is only used in 関西弁.
The kansai 「わ」 is used by both male and female, and has no feminine effect even when used by female. The feminine 「わ」 does have an feminine effect, and is sometimes used to signal that the character in a book is a female etc. The two わ are also pronounced differently.
Interestingly, 「わよ」 is exclusively feminine 「わ」. Male usage of 「わよ」 is associated with transgender and homo-sexuality (talents like 美輪明宏 use it, presumably to emphasize their femininity?). Usage of わよ seems to be rare among Japanese female (at least in younger generations).
So to summarize, Japanese female mostly use something other than わ nowadays.
As an anecdotal evidence, I'll cite this chiebukuro question:
ドラマや漫画（アニメ）に出てくる女性はほとんどの人が
語尾に
「～わよ」「～よ」「～わよ」「～だわ」など
（例・「知らないわよ！」
・「だからアホなのよ」
・「そう思ったわ」
・「おちゃめだわ～！」）
つけますが、今までこういう女の子言葉使う方実際には見たことがありません。
For people's opinion on this, you can take a look at the question (many obnoxious comments unfortunately, but there you go)
In general, sentence final particle use varies a lot depending on the region. In Kyoto, where I live, for example, men and women both use わ freely, and even throw out the occasional 「わよ」. I think it sounds softer, more restrained and less insistent than よ but that's only my personal opinion. I haven't seen a thorough breakdown of the usage for the two, but I think you'd have to dig through academic papers to find something. I've been told that the meaning is essentially the same.
