Something I look for, especially with racial class progressions, is to see whether stat bonuses make up for lost hit dice, BAB, skill points, etc. I keep in mind that if I get a +2 Str bonus (or +2 Dex for ranged attacks) for each LA increment, then I effectively recover a fighter's BAB for those missed hit dice. Con increases, if they get up to +4 or so, can also effectively make up for hit points.
Here's how I analyze, say, the weretiger racial class progression from Dragon #313 (as a "for instance"):
It's ECL 9, with only 6 HD. The final BAB at level 9 is +4. The total Str bonus, however, is +12, and the Dex bonus is +4. Therefore, the total modified melee attack bonus is +10; out of 9 levels, that ain't half-bad. Our total modified ranged attack bonus is +6; again, out of 9 levels, this is respectably along the lines of a rogue, cleric, or monk.
Now, we are also missing 3 HD from our class progression. We have a +4 racial Con bonus, which means that by level 9 we have an extra 12 hitpoints. We calculate what we lost to LA using the weretiger's hit-die type: d8. Three hit-dice of d8 yield 4.5*3=13.5. This means we lost 1.5 hitpoints due to LA. I consider this acceptable (especially considering we gained a modified melee attack bonus modifer).
Now we look at armor, since our weretiger won't be wearing any (like a good many monsters in the MM). Keep in mind that we already have +2 Dex modifier accounted for. We throw on top of that a +4 natural armor bonus for a total of +6 (unless we're in animal form, in which case our size modifer takes our AC downa notch). This, frankly, is not very good; not at level 9, anyway. If we have a nice DM, however, he will let us craft bracers of armor that we can wear in hybrid form. This can be done for most other creatures, as well. This brings us back up to +10, which is acceptable for a 9th-level character. Not stellar, but adequate.
Slap on all out other goodies, however, and the weretiger is, mechanically, a very nice race to take for fighting.
The only weak spot is skills. Only 6 hit-dice, with only 2 + Int mod skill points each. This is offset somewhat by racial bonuses to balance, hide, and move silently, but the character is still skill-hobbled, IMHO. Still, the race is quite compelling from a combat mechanics standpoint.
This same analysis shows that a good many (but not all) PC-playable creatures in the MM are viable when stacked up against a PC of the same level with normal classes.