Pros: Quick, but extended relief. Powerful, effective medication.
Cons: Somewhat pricey, must follow very exact directions to work effectively.
My wife developed a very bad cold with a dry hacking cough in her throat. She used Delsym 12-hour syrup and 5 hours later she was hacking again. Three days later, when NyQuil and TheraFlu did not work, and it moved into a chest-congesting mucus cough (the sore throat had disappeared), every pharmacist we asked said Mucinex DM would work so she tried it. (Note that this is for chest coughs that produce phlegm, not for sore-throat related "dry" coughs).
Less than an hour after taking it, she was coughing so hard I thought she was going to vomit. We think this was the medicine starting to do its job because at the 7 hour mark, she barely had any cough, just tightness in the chest from the cold itself, not the medicine. Note that this is from taking 2 tablets, not 1, which it says on the bottle you can do and I strongly recommend. You also need to make sure you take it with a full glass of water and not disintegrate the tablet in any way because that can make the timed-release effect go awry. This is one of the few cons I can see with this because it is very easy to miss this.
Yes, it may not last the full 12 hours, but at least she's not coughing so bad I feel like she's going to vomit or end up in the hospital. Our almost 6 year old son joked to her today, "why are you not going in to work" after calling in sick 2 hours earlier, as he thought she was that OK now.
It is a bit expensive (about $20 for 40 tablets), but definitely well worth every penny. It is also sold for about $9.95 for 20 tablets, though this size is somewhat hard to find. It is definitely the best non-prescription cough medication we have ever seen and we highly recommend it.