Like any ccTLD, the buyers will almost always come from the host country.
But Iceland is a very small market (about 330K inhabitants). Therefore sales are going to be rare.
If you have stellar keywords (Icelandic keywords) for which there are local end users, go for it. Otherwise stay away.
On the occasion, .is is used for domain hacks but there are plenty of other ccTLDs used for the same purpose. But there isn't much of a market for domain hacks.
I have sold LL.is, and have had inquiries about other generic (Icelandic) keywords but end users generally submit low offers (< $1000). Because it is a small market you shouldn't expect .com prices anyway.
On the plus side, the TLD is stable and trustworthy, and the fees are moderate. Domain transfers aren't difficult if you know what you're doing.
PS: a few years ago, there was a Russian citizen who owned 100+ .is domains. According to my research he was the second biggest .is holder. Last time I checked he had dropped almost all of them. Must have lost a lot of money. ccTLDs are specialized markets and it's very easy to waste money if you don't understand the local trends.