Alarm

Abjuration
Level: Brd 1, Rgr 1, Sor/Wiz 1
Components: V, S, F/DF
Casting Time: 1 standard action
Range: Close (25 ft. + 5 ft./2 levels)
Area: 20-ft.-radius emanation centered on a point in space
Duration: 2 hours/level (D)
Saving Throw: None
Spell Resistance: No
Description

Other Information
Wards an area to trigger an alarm (mental or audible) when a creature larger than tiny size enters the area and does not speak a specific password. A mental alarm wakes you from normal sleep if you are within 1 mile of the warded area; an audible alarm can be heard by anyone within 60 feet of the warded area. Ethereal or astral creatures do not trigger the alarm.

Pros:

Long duration: by 4th level a single casting will last an entire 8 hours of rest.

Mental version of spell gives no outward sign of alarm. Someone sneaking up has no idea that have triggered an alarm.

Cons:

Audible version somewhat negated by a Silence spell (creatures inside a Silence spell do not hear the ringing noise).

Tricks/Combos:

Use it to detect creatures following you. Cast it on a doorway that someone pursuing you would have to enter, continue onwards and wait. If the Alarm spell triggers, this gives you time to prepare an ambush or other welcome.

Overall:
A good spell to protect against being ambushed while resting. Even if your party takes shifts on watch, Alarm will save your butt when the watch gets garrotted or kidnapped silently. The ability to use it to detect pursuit is nice. A rogue sneaking back could accomplish the same task, but Alarm lets you do it from a safe distance away. It can also be used to detect intruders entering your home. If your party is breaking into a wizard's home, expect one or two Alarm spells to have been silently triggered.

Recommended for Bards?
Depends. Since you don't have to prepare it, you aren't really wasting a slot each day on the chance you'll need it. It's a perfect way to use up an extra, unused spell at the end of the day. There are other 1st level spells that might be more useful on a day-to-day basis, but the simple fact is that an Alarm spell while resting is never a bad idea. However, Bards get only a handful of 1st level spells. Alarm is okay, but chances are there are other spells you will need more.

Recommended for Rangers?
Yes, depending on area. If you are in a heavy combat area, you should use your few spells on combat buffs instead since there's a good chance you'll need them. If however you are traveling on main roads or in relatively safe regions, Alarm is a decent choice.

Recommended for Sorcerers?
Depends. Since you don't have to prepare it, you aren't really wasting a slot each day on the chance you'll need it. It's a perfect way to use up an extra, unused spell at the end of the day. There are other 1st level spells that might be more useful on a day-to-day basis, but the simple fact is that an Alarm spell while resting is never a bad idea. However, Sorcerers get only a handful of 1st level spells. If you're a blaster Sorcerer who throws a Magic Missile every encounter, take it. If you're a utility Sorcerer who likes having lots of different spells to use, then you might need the slot for a better spell.

Recommended for Wizards as an initial spell choice?
No. When you are low-level, it won't last long enough to be used as a safety alarm while resting. It's a good spell to have scroll-scribed by 4th level though, when it will last 8 hours. It's a tough decision to memorize Alarm rather than say, Mage Armor or Magic Missile, but if you're adventuring in dangerous areas it can be worth the sacrifice.

Recommended as a scroll?
Yes, but only for the "detect pursuit" trick. A scroll would have to be at least caster level 4th to last an entire night, and burning one each night as a precaution is just wasteful. If no one in the party can cast Alarm, just take shifts on watch and hope it's not the night the silent assassin comes calling.