What can I say that hasn't been said. Follow the directions, use a surfactant, and in a week, you will have white weeds...

Update:

if you have St Augustine grass, and you follow the instructions to the letter; you WILL damage the grass around the weeds. It has been two weeks since the application, and the weeds are dead. Tenacity did it's job, but the grass around the weeds has been removed of the chlorophyll as well.
My neighbors were very surprised because my lawn is usually one of the nicest around.
The white areas will grow out as the bottom of the grass blades are still green and actively growing, but your yard will look sickly for about a month after the application, just so you know what's in store for your lawn.
Overall, I believe Tenacity is a great product as it definitely does what it is suppose to do, but there is a price to pay for ridding your yard of weeds. That is, your yard will look bad for about a month after the application. And....it absolutely SMOKES crabgrass and nutsedge. Here's a photo of my yard 2 weeks after application. If you can live with this, give it a try. If not, then you're most likely going to have to mechanically rid your yard of the weeds (pull them up by hand).

Works well for spot spraying and does require a second treatment in 7-10 days to really do the job well. Be careful to know if other grasses in your lawn might be effected. I didn't really care as I've had a growing problem with crabgrass for over 10 years and found nothing of use until Tenacity. I water regularly so no portion of the lawn goes dormant when dry season arrives. Inexpensive and very effective in my opinion!!

Easily eliminated unwanted grass varieties for a more uniform and attractive lawn.

Update...totally works. I'm about 7 weeks I think from my first application. OH BOY, I thought I had made a mistake!! My beautiful lawn turned splotchy white. I thought I had destroyed my lawn, but no...the lawn came back nicely with very little lime green still present, where it had huge clumps previously. I am now a believer in Tenacity. As a further benefit. In my back yard I have a small section of grass, basically our dog's bathroom. I don't do much to that at all but I applied Tenacity there... Got rid of the Poa Triv and Annua...and the Nutsedge!!! Looks GREAT!!

I take pride in my lawn. 3 years ago, that little bit of lime green grass showed up. 3 years later, it was all throughout the lawn. I had both Poa Trivalis and Poa Annua. I've spread activity with Tenacity over 3 weeks. #1 was specifically targeted at obvious big splotches. #2 I did a generic spray of the entire front lawn. Took about 2 weeks and the white undesireable grasses are very obvious. 3 weeks later I repeated both actions (spot spray) and total lawn coverage. It's August 31, I'm going to wait a few weeks and then core aerate and over seed.

So far, it seems to work. I'm sure it will be an ongoing effort to keep the Poa(s) at bay. Crab grass, clover and dandelions, not an issue this year. I've got a bit of NutsEdge. Need to read a bit and see if Tenacity takes that down too.

Looking forward to the darkest of greens next year.

Well it works on creeping Charlie in the fall haven't used on much else but I have great plans for this and know it will do its job.