Firefoot is a beast of a mount. Generally, he has very good synergy with Éomer. That is only fair considering the lore. With that being said there are plenty of other options such as Thorin Stonehelm or many of the other heroes. There are plenty of other Rohan targets as well depending of the situation and trait-paring.
When I look at attachments I always examine what I'm getting in terms of stats and abilities in comparison to cost. A fair price is 1 stat/ability per 1 resource. Firefoot not only matches that, gives you more than that with Éomer with the extra boost. 2 resources may be steep if you are only running one hero without a Forth, The Three Hunters! contract. Be that as it may I do believe Firefoot is cost effective for what you're getting. What I really appreciate about Firefoot is how well it can make use of his stats and ability as well as how the ability shines in combat-heavy quests. Not to mention combat is an essential part of the game. Now, back to Eomer for a moment. With Firefoot as the only attachment on him it is easy to get Eomer up to 7 thanks to the discarding Rohan synergy. That is often enough to destroy one enemy. Firefoot's response will heavily damage another enemy, with a strong chance of destroying them as well. And that is one attack (which must be done alone, mind you). Your other characters will easily finish off a wounded enemy. In most cases this straight up eliminates the pressure of combat. A similar situation is true with high attack heroes, or other modifiers to raise attack/lower defense (Grimbeorn the Old, Thorin Stonehelm, Gúthwinë and MANY other attachments).
While the stat boosts are almost always helpful the response is situational. You need two enemies engaged with you to make the most use of it. Or, you could get creative and use Forth Eorlingas! to strike once in the staging area and dish out the direct damage to an enemy engaged with you. Bonus points for theme! It is of course restricted, and the choice of hero options are limited to Tactics and Rohan conditions.
Still, the stats alone do pay off and the response is a huge bonus. Really what you're getting is much more than 2 resources worth of an attachment. That is rare and quite valuable. Let's not forget the bonus amplifies with trait-sharing Elfhelm AND it is searchable with Westfold Horse-breeder. The only time I am ever indifferent to topdeck this card is if I have one in play already. Even then I'm glad to have a backup. All things considered this card is a real bang for your buck. Plus, it sports lots of theme! And you can't beat that in a game like this.