They're great games, totally different from any of the GW games. The games are fast paced and deadly, as while everything can cause a lot of damage, fewer things can take much (Khador being something that can). You have a lot more ways of doing things, with a lot more special abilities and spells, and the feats can turn the game on its head very quickly. One of the things I really like is that you can be completely dominating the game, but if you slip up for a second, you can lose from a very strong position. Games are very rarely over until they are over, you really have to stay switched on.
However don't be fooled into thinking it is simple, or that the rules and minis are perfect. They're not. The rules can be complex and slow the game down until you know them well, as well as the fact that they are constantly being FAQd, changed and clarified (although these are all freely downloadable from the official site).
Warmachine also suffers from the fact that Jacks--my favourite part of the game--aren't very competitive. You can do alright with them, but you're often better off without them (aside from some of the little ones). That's why I like Hordes better, as you *have* to take the big warbeasts, it's how your Warlocks (your most important model) get their power. This is almost opposite of Warmachine, where your jacks get their power from the Warcaster.
The minis also suffer from flash and mold lines as bad as GW, but can be even harder to assemble. And everything is metal.
But don't let me put you off, it's a great system and I highly recommend it, but don't let anyone convince you that it doesn't have the flaws that GW games have.