@AimeeD In TS3 I had a plethora of custom content (CC). I really liked how I could use the in-game tool to change the fabric type, color wheel for colors, etc. Although I downloaded a TON of content, I never had any issues, though I'm sure there are plenty of mischievous people out there who might insert a virus. I do keep my virus scanner up to date. Originally I decided I probably wouldn't get CC for TS4, but without the color wheel from TS3 and the customizing tool where I could use different fabrics for furniture and clothes, I've found myself quite bored with the selection. I now have almost a gig of CC for TS4, but I carefully organize it and review it. I get most everything from TSR (the Sims Resource). I haven't had any issues yet, but I sure do enjoy using all the custom colors and different clothes.
One of my favorite things to do in this game is create unique, efficient houses. With the in-game content, it's hard to make them unique AND look good if you're making more than a couple houses. I like them to be distinct. So CC is a must for me at this point.
If you decide to use CC, my advice is to keep your virus scanner up to date, review every download, keep a special folder outside your Sims game where you keep it all organized and in folders so you can easily find and remove things you don't like or that look funny, etc. Copy what you want into your Sims mods folder and then be sure to remove the mods during patching time. CC is such a huge part of the game for me. I would be bored without all the extra colors, hairstyles, etc.