Ian asked my opinion on this question (I helped with the playtesting), so I'm going to throw out some thoughts on the subject.
*Important note* The Book of Templates was not designed with PC's in mind, so some templates, as written, simply can't be balanced properly (anything that doubles HD comes immediately to mind). That said, let's look at a couple of them:
Enchanted
SQ: Detect Magic, at will
SQ: Read Magic, at will
SQ: Light, at will
SQ: Mage Armor, ChaMod/day
SQ: Invisibility, 1/day
SQ: Immune to sleep, paralysis, charms
SQ: Low-light vision
SQ: SR 10 + 1/2 HD
Stat: Charisma +2
I really like this template for the flavor it provides. Concerning balance, let's assume you apply it to a 1st-level sorcerer. The spell-like abilities are probably a +1 ECL on their own. The immunities and stat bonus are another +1 ECL. The SR is AT LEAST a +1 ECL. So right there, you're looking at a 4th-level character who (with the exception of the spell-like abilities) only has the HD, spells, and skills of a 1st-level sorcerer. His defensive abilities, however, are outstanding for a 4th level character. He is immune to many spells and spell-like abilities, and has a SR of 10, meaning a 4th-level opponent has a 30% chance of not affecting him with a spell. Or, try applying this template to a paladin - she'll love the Charisma bonus, the immunities, and SR. The spell-like abilities are less useful, unless the paladin eschews armor. Assuming either character survives the early levels, +3 ECL seems about right.
Now, one for the fighters:
Ebon Servitor
AC: +4 Natural Armor
Stats: +4 Str, +2 Dex, +4 Con
SA: Smite Good 1/day
SA: Damage Feedback
SQ: Detect Good, always on
SQ: Bonus feat
Whoa!! Try applying this to a 10th-level barbarian and see what happens. The natural armor is +1 ECL, the stats are AT LEAST +2 ECL, Damage Feedback is probably +2 ECL, the other stuff around +1 ECL. We're looking at about +6 ECL total, but if you run a 1st-level fighter with this template with a group of 7th-level characters, he's toast. So let's knock it down a couple levels. After he gains a few levels, you'll have a fighter who'd be tough to take out in a stand up fight, but with even lower saves than the normally lousy Ref and Will saves a fighter gets. Magic will be his achilles' heel. And how in the world do we balance Damage Feedback? Imagine a 10th-level character protected by a Shield Other spell... We might have to split the difference and go with +5 ECL. It's a tough call. I'd personally be very reluctant to allow a PC to take Ebon - or Argent - Servitor.
As the book says, even the CR adjustments need to be taken with a grain of salt by a GM - even more so with ECL's. I'd feel comfortable starting with the ECL's listed above, but I'd also ask any player using one of the templates to remain flexible - if an ability becomes too problematic, it might have to be changed to keep the character from overshadowing the other PC's. In the end, while Silverthorne will publish "official" ECL's, the GM always makes the final call.
What do you think, Leopold? About what you expected? Or do you think we're too high (or low) in these calculations?