::nods:: Not all hit dice are created equally. A HD of Dragon (d12, Fighter BAB, THREE good saves(!), and some pretty good creature-type based immunities and perks) is practically worth a level in and of itself- a HD of Humanoid is barely worth anything at all. ECL can be useful, but the whole "ECL=HD+LA" formula is extremely dubious to me. A level isn't just the HP, BAB and save bonuses you get with each new HD, it's also the various other perks that go along with it. If you don't have some way of balancing this out, monster characters get hosed.
Of course... that may be the whole point. I've heard (secondhand, so I'm unsure of the reliability of my sources) that some of the designers at WotC have admitted that the ECL formula is designed to create monster characters who are weaker than their equivalents so as to discourage people from playing anything outside the core races/ECL 0 races. I don't truck with the reasoning that the potential extra work or interesting roleplaying or compensating the 'core' races is an adequate payoff in and of itself- What may be 'weird for weirdness' sake' in one setting might be perfectly normal in another. Maintaining game flavor by vetoing 'weird' or otherwise unsuitable concepts is the DM's job, not the game designers, and by not explicityly spelling out that that reasoning went in to the formulas, the ECL=HD+LA equation creates a lot of players and DMs who are expecting the formulas to work as advertised and they WON'T. Even if I assume for the sake of argument that "weirdness" as a balancing factor is a good idea, it's still a balancing factor that should have been identified to players and DMs rather than only being revealed when one game designer or another chooses to mention it.
(CRs for the dragons in the monster manual are another thing with a similar unwritten assumption that causes problems, but that's a separate rant.)
On the other hand, DON'T use Upper_Krust's CR formulas for this, no matter HOW good they are! ECL is not equivalent to CR, even though a character level is supposed to generally be worth a full CR point when you're playing an NPC properly. Some of the abilities that a monster has may be FAR more useful to them when they have a chance to use them repeatedly as a PC than if they're used in a one-shot encounter. At-will telepathiy is handy for a monster, but potentially balance-altering when used by a PC. A casting of Wish once a year comes up a lot more often when you're fighting different monsters of that kind in several encounters, but is less of a big deal when your PC will probably use it once in the entire campaign. Flight adds to an encounter's challenge, but allows PCs to bypass many otherwise-challenging encounters entirely. Eyeballing is imperative.
I agree with Ottergame- if you're going to stick to the WotC formula, you're best off with melee characters, as you're missing out on less from the ECL. Skill-based classes are basically worthless, and caster-types are probably a bad idea unless you're playing something like an ethergaunt which has "as a X-level caster" built in. Savage species is also a feasible workaround if you're willing to wait for some of the powers and it doesn't conflict with your vision of the monster. (Ie, I dislike the Savage Species illithid because a flayer that has to wait that many levels to be able to eat brains would starve to death)
That being said, if you're willing to look elsewhere, there ARE fairly good formulas out there which do a decent job of balancing monsters against standard classes. I personally use
Soldarin's ECL Calculator and so far I haven't had any problems with it. It's also flexible enough to help guesstimate for variant monsters who differ from the listed stats, completely new monsters and races, and templates that might be worth more ECL on some creatures than others. I find it a good starting point, especially if you're still playing 3.0 like me and don't need to tweak it. Also, like any formula, it's thrown off if you deliberately make suboptimal choices at character creation- like multiclassing, some combinations are a LOT more effective than others.
From personal experience... ECL is easier to handle when you're coming in at a point where you can 'pad' it with a decent amount of class levels, rather than just riding it raw. Also, it's important to play the monster character to its own unique strengths rather than expecting to run it like a standard PC. I'm playing an illithid bard in a Planescape campaign at the moment, and my combat tactics aren't something I'd ever use with an ordinary bard. (For example, with my good Dex and Weapon Finesse:tentacle, I'm VERY good at dealing with enemy casters) Make sure to analyze your monster abilities and figure out how to best use them to your advantage in and out of combat, and make sure you don't forget them (like SR or immunities). Since you're paying for them in levels and feats, you should get as much use out of them as you can.
Edit: Curses, MoogleEmp and Kamikaze made some of my points not only before I did but more eloquently. Sorry for the redundancy.