Give them what they want, what's useful, what's entertaining -- in that order.
Look at it from the other side: if you're even remotely internet savvy,
your inbox is layered with filters that shoot incoming mails to different folders. Anything with "SALE", "FREE", "Trial" or even "Work" gets shot down to the trashcan. Any email you send to your
subscribers with these keywords can get you filtered out, unless you prove from the start that what you write is worthy to land in the inbox from the start.
Things to bear in mind: Speed, succinctness and impact. Important information comes first, in easily digestible chunks that explain its importance.
Things to balance: frequency, relevance and entertainment. Short, sweet emails that share new information in a bulleted format can come with more frequency than longer, chattier newsletters.
Above all: value, value, value
People are all too often pressed for time to read through everything that lands in their inbox. Your e-newsletters should be stick to giving useful information about your products (tips and shortcuts, newbie advice), late-breaking industry trends and developments etc., things you believe will only help your subscribers.