Peercoin was the first coin to employ proof-of-stake (technically hybrid PoW and PoS) and was considered to be very innovative when it was first launched. Technology-wise, other coins like NXT and BTSX have surpassed it but it has one of the best and most highly-respected devs (Sunny King) working on it and a substantial community behind it. And unlike many other older coins (e.g. Namecoin and Litecoin), there are still new developments being built on top of PPC too such as Peershares, and NuBits/NuShares.
The goals behind Peercoin also differ from that of Bitcoin and Litecoin. Peercoin isn't really designed for small transactions due to its fixed transaction fee. It's not really a currency that you would buy baked beans at your grocery store with, for example. It was always intended to be used as more of a savings account or a "backbone currency" (as Sunny King puts it) than a currency for day-to-day transactions since the very beginning.
Very true. And BTW OP, perhaps invest in more than 1 type of coin. BizMark brought up a good point with the Nu network. NuBit has a good idea with keeping value equal to a fiat for purchasing with ease. Its managing service, NuShare, is the one that gains/loses value based on the Nu network's success. Maybe a good investment here, too.
But back to PeerCoin, it has been laying low for a while and isn't a casual alt-coin. Biz explained it well, and I have nothing more to say about it. It may prove to be a good investment, but trends show a decline in value. For that reason, I have to advise against it. But don't take my word supremely, make your own decision. Invest at your own risk, and don't invest more than you can afford to lose.