Friends sets are adorable and they're partly responsible for me getting back into Legos.
When I saw the theme at first, I thought they were cute, but thought it didn't really fit in with regular Legos. Around the same time, a friend at work built his own modular house and I'm like, that's so cool, Legos are awesome. I grew up with Legos and matchbox cars and became an engineer. So looking online, I bought some regular sets for myself. Then I got the Friends Horse Stable for Christmas. Haha! I wasn't sure about it, but opened it and built it and the thing is soooo cute. For some reason, I thought the minifigs of Friends were a lot larger than the traditional minifigs, but that's not true. They coincide just fine. Different, yes, but can still work together.
What's more, I adore the unique colors and shapes and such available in the Friends sets. When I think of things I'd like to build, I really like the opportunity to use brighter colors along with the traditional colors.
I'm a horse person, as many girls are, and the horses are probably the best ones Lego has designed so far. What's interesting is the many different versions of horses over time.
These things are a hit with girls, but some of the price points might be an issue for parents buying for kids, if they're being marked up in the future. For investment, it would have to be well-to-do persons buying for kids, or other adults. Not sure how that's going to play out in the future but if anyone is looking for the parts, there is that too.
I'm buying to collect for myself with an extra set here or there, just in case.