They're super convenient, but on their own they're kind of...bland! Thankfully though, Japan has a whole host of ways to make those bland IC cards cuter -- and the easiest option has to be stickers!
I decided to get a Rilakkuma sticker for my Pasmo card, because I just loved this design. I ordered it off Amazon Japan, but they do sell them in stores at a lot of places here - particularly toy stores like Kiddyland. Here's some of the ones though that they have on Amazon Japan right now...everything from Disney princesses to Star Wars!
The stickers will set you back around 300 to 400 yen, generally - and if you get bored with a particular design you can easily peel it off and replace it with another one (they don't leave any sticky residue or anything like that). They don't affect the functionality of my Pasmo card at all, it still scans the same way as it did without the sticker - so I'm sure you could even buy these and use them to decorate other types of cards that you have.
If you're going to be visiting Japan and you're not planning on getting the Japan Rail Pass to get around, purchasing one of these IC cards is a really good idea. Not only does it make for an interesting and unique keepsake, your train fares are cheaper than buying individual paper tickets, and you don't need to worry about calculating the cost for getting from point A to B - just charge up the card and away you go!
*Note: if you don't want to keep the card after your travels in Japan, you can take it back to a Pasmo retailer who will cancel it and refund you whatever the balance is on the card and the 500 yen deposit for the card itself (minus a processing fee). Details are here.