Since Patch 1.1.2, the Turbo Reaper has gone away.
What are pros using the Reaper for these days? Just an early scout?
Early scout, mid-game scout..
They are also still good for harrassing the mineral line, especially on vertical terrain.
When you get +1 weapon upgrades, 2 reapers one-shots the workers.
So you can easily do some serious damage with just 2 reapers if you can manage the surprise the opponent.
I won some good games with just this tactic. Sneak 2 reapers, kill 6-8 workers in seconds and voila!
Reapers are still effective as anti zergling/zealot/marine units, in any situation where they are not easily reached by the enemy. They are particularly useful vs. tier 1 melee.
Reapers do tremendous damage to light units. Unfortunately, their range is short, they have few hitpoints, and they like to race ahead of your army. Consequently, when mixed into a terran mass, they run to the front and immediately die. Reapers only work in situations where you can afford to manage them heavily.
In team games, hard, early tier 1 rushes are common. If you scout that your opponents are planning one, padding your army with reapers can be effective. They are available faster than siege or other defense alternatives, and have good mobility for defending your partners.
You can assume that the opponents plan a double tier 1 mineral-heavy rush if their early bases fit the following profiles-
If you scout this situation and have not been planning a hard early rush yourself, you'll need to quickly change gears to deal with it. A few reapers will cut through a force like this quickly, if the reapers can avoid direct contact with the enemy. They can either be placed in a bunker if you need to defend your own base, or used on the outskirts of the battle, trimming the edges of the enemy if they try to advance on your partner.
If it the opponent starts to heavily mix in roaches, stalkers, or marauders, send your reapers out to harass peon lines, and switch to marauder production. Reapers should never, ever engage armored units.
For early game timing attacks send the Reaper to harass the mineral line, then send your main army and attack the front.
Often players will send too many units to deal with the reaper and you can more easily kill their main army. If they don't send anything, your reaper can do massive damage on the mineral line while your opponent is busy.
Reapers can also be used to get line of sight up a ramp, useful for if your army is blocked by a strong wall-in or forcefields, or you're trying to siege tank his base and can't see the highground. Also handy in 2v2 for helping your teammate warp in uphill etc.
Reapers can be used in a variety of ways, I tend to use them for fast worker harrass and scouting xel'naga towers. Also due to there quick speed a do use them to attack expansions of the opponet and lock them in there base. Overall reapers are good early to mid game for harrasment and scounting, but in late game zerg and toss tend to have ranged units e.g. blink stakers and mutas which make them less effective.