I've not heard anyone complain about LJ's security (assuming you trust the admins), and the reliability is OK. If you have a paid account they are kind enough to give a week's extra free membership if they have a bad outage. The good thing is you do not have to maintain a server. The bad thing is you have no control over their servers. The cost of membership is $25/year (or about 25p a week) and includes some photo storage.
There have been a "couple" of problems with LJ security over the last seven or so years that I've been using it - but all of them were fixed within a couple of weeks, which is great. The three strands of payment, free, advertising and paid for are fantastic- if you just want blogging, the free one is perfect, if you want the extras then the advertising gives you some of them, where paid gives you a few more - I personally have a paid-for account where most my friends are on free ones.
LJ does allow you to shut out, control, screen and IP address track public, friends and groups of friends to varying degrees. For example I screen and IP address track anonymous comments on my account, and I tend to make friends only posts about personal stuff. LJ supports OpenID, which is taking off, and this allows you to add people with OpenIDs to your friends list so they can read your non-public entries.
Not just that, I believe that Brad Fitzpatrick was one of the founding people for OpenID (Brad used to own LJ but sold it to SixApart (Wordpress guys)). The different options can be set by default and indeed overridden on any post.
Your non-public entries are not visible to search engines, and you can discourage the well-behaved search engines from indexing the rest of your journal too.
A few years ago, there was a massive rant when Google indexed some LJ posts which where had anti-bot things on them. They had a fight with google and google said sorry (iirc).
Of course an RSS feed of your journal is available. You'll spot 2 or 3 LJ of us LJ users carried on http://planet.alug.org.uk/
RSS of your journal, your friends and also a FOAF. There are also funky XMLRPC interfaces that you can use to make posts and get post information back, and a bazillion plugins to help you make posts (like deepest sender plugin for firefox).
JT