Odd that noone has mentioned this yet - but the Harm spell hasn't really changed since 1e. The difference is that in 3e any touch spell stays "loaded" until discharged, so that even if you miss - you don't lose the touch attack.
In the past you cast the spell, made your attempt, and if you missed that was it. Now, you can cast it several rounds early, wait for your opening.
The big advantage is that adventuring parties usually have the wizard and the fighter to set up a simple one-two punch with the cleric casting harm.
People always use dragons to illustrate that this spell is not broken - but keep in mind - there are a lot of powerful creatures that do not fly or have magical resistance. Against a single powerful creature the harm spell gives a party a huge advantage of victory with little risk - that is the key: high chance of success - low risk.
I use the following rule: If the target makes its saving throw it loses half its current HP.