I don't usually plug products much, but I found Campaign Monitor to be one of the better ones I've used so far. The thing I like best about it is its pricing structure. There are no setup fees, no monthly fees and no hidden fees. For each campaign sent with more than 5 recipients, it's a flat delivery fee of $5 plus 1 cent/recipient. So it's a purely pay-as-you-go product.
Sending newsletters by email is one of the most effective way to reach one's clients/audience, whether your mailing list is in the tens or in the thousands. You have to realize that some will not reach their mailbox destination, but that's a function of anti-spam filters in email providers.
Using Campaign Monitor is not difficult, but because it's built for designers, it gives its users complete creative control over the design and structure of their emails. You can use their templates or you can create your own to send in HTML, plain text or both.
I use my own newsletter design (the link to my latest newsletter is on the right of this post, under "My Other Websites". My mailing list, which is expanding every day, is all permission-based (as it should be) and I'm ruthless in weeding out those names who signed on to the list just to get details of my photo-expeditions' itineraries for their own purposes...a bunch of those have already been removed!
It's one thing for photographers to keep track of what others are doing (some view it as 'industrial espionage'...but I don't) and quite another to essentially 'steal' photo itineraries which have taken me effort and time to put together, and just walk away. So I'm very selective as to whom I send itinerary details to.