The mount is worth the investment. Consider that a large portion of your leveling and questing time involves travel to and from the quest objectives. The amount of time saved in the long run is pretty surprising, even just from a 20% speed increase. Not to mention that you can avoid getting run down easily in unwanted PVP with other pre-60s.
But seriously, get the mount. Save up your money as you level, and DON'T buy gear. Once you hit level 58 and get a few quests in Outland complete, you'll be getting armor and weapons that make the stuff you're wearing look pathetic. With some good gathering and auctioning, you should be able to afford your epic mount in your early 60s (gold flows pretty quickly in Outland). Buy the epic mount and don't look back. Again, the quest rewards should keep your gear pretty up to date, and there's always instance runs (and dungeon mission rewards) that should keep you in good shape.
You will need your epic mount for leveling to 70. Unless you love getting chased down and ruthlessly slaughtered by bored 70s in Hellfire, or spending forever trekking around, it's easily worth the expense.