Steampunk is more than an aesthetic: it's the setting of cogs, gears, brass and steam, but it's also the attitudes and social conventions of the people found there - frequently a kind of alternate Victorian era. There are novels that are clearly steampunk, and those that are only ambiguously so - Kultus being one of the former; Geist one of the latter.
But one key question is left unanswered (aside from when Datamancer will make another of those all-too-awesome laptops): why should you read it? Let me fill the gap...
- The trappings. There's something very beautiful and uniquely exciting about steampunk's trappings: the brass, cogs, and steam from which its technology is (almost universally) composed. Yes, it's just a fun factor - but let me put this to you: no other subgenre has this many airships. So while the common features of the setting might be fun in themselves, they also lead into sequences that can really only belong to steampunk. If that seems entertaining to you - well, you might want to give this subgenre its chance.
- Intermediacy. Most fantasy is either set in a quasi-European variant of the Medieval period (or at latest Renaissance), or in the modern day (urban fantasy). Steampunk is an interesting midpoint between the two, and allows an interesting mix: no instant communications like UF, but a more developed and industrialised society. Of course, it's also part of speculative fiction. So if a story between urban fantasy and epic doesn't entice you - well, steampunk also mixes in some of the trappings of SF. Giant clockwork robots, anybody?
- Society. Steampunk is generally based on an alternative Victorian period, which is fairly rare in fantasy - and fairly refreshing. It also affects the characters, who will have different prejudices, different attitudes, and different occupations - all of which can lead the plot and characters blissfully far from the usual. Furthermore, economic development also plays a part: in steampunk, more characters can realistically have the time and freedom to take part in the average high-stakes fantasy plot. Of course, the era was still hard on many - but compared to your average serf, there's no contest.
- One final point? Who's writing it - to which the answer would take at least a page. Suffice to say, there's a lot of great authors writing steampunk right now, so why not give it a chance? From Richard Ford's recent debut, Kultus, to the more established series - if you haven't tried steam, there's a lot you could be missing out on.