Bah, I typed a reply and seem to have accidentally deleted it! I am a few islands south of Lewis, and I have lived in Stornoway for short periods.
A lot depends on the area. Stornoway is positively metropolitan compared to some of the outlying areas, and has most of the things you would expect in a small town. Even Catholics like me feel much safer there now, which is huge progress (years ago, the one Catholic church in Stornoway burned down and people were shocked to hear that it wasn't arson!). Religion is a Very Big Deal. And as religions go, the predominant one is strict and (to an outsider) joyless. But it's there, it has great influence and you just have to work round it.
Having said that, crime is relatively low. You have to keep an eye out for things going on (sales of work, toddler groups if you have young children, craft groups, anything!) and make an effort to go along and meet people. It doesn't take too long before people get used to seeing you and you will gradually become part of the community. I wasn't born here, but arrived 35 years ago with my parents. There are things I miss about the mainland, and one day I may move back, but I am definitely part of my community. When we are in need, the people around us rally round and help. We do the same for them, and it all feels very supportive.
One thing that can be really problematic is the food shortages which can happen when the ferry doesn't get in for extended periods. I have a polycrub which enables me to grow enough fruit and vegetables for my family, and we have a large vegetable garden. Not everyone can do this, obviously, but it has sometimes meant that we have fresh food and others don't. (We share, obviously!).