I've used a Samsung F1 750GB as an OS drive for about 2 years now, and during this time some SMART warnings have cropped up. Here's a picture:
The raw read errors have been around for about a year, but the uncorrectable sector count grew a few weeks ago. The seek time performance used to be 0, but has grown in the last few weeks.
In addition I've noticed that whenever I do a full virus scan, it creates a few "disk" events in the event log, stating that there is a bad block. This is confirmed by the error scan in HDTune, it finds 2 red blocks. The drive's quick self test fails due to a problem in read element.
I know that the drive has started to fail, but how long do you think it will last? Since backups are in place, I'm still using it as my main drive, as it seemingly works just fine. HD Sentinel estimates that it has over 800 days left, but I don't know if I should believe that.
When the drive starts getting read errors on a sector it tries to re-read the data until it manages to do so successfully, and then it writes the data into a "spare" sector and stops using the damaged one. If the drive has done that, then you didn't loose any data and the "Off-line Uncorrectable Sector Count" goes up.
But if the drive isn't able to successfully recover the data from the sector, it marks it as a "Current Pending Sector". Every attempt to read that sector will cause the drive to attempt to read the data, retrying multiple times. At the end, if it couldn't get a clean read, you get the disk event you saw when going the virus scan.
So those "Current Pending Sectors" you see are blocks that contain something that the drive can't read - basically it's lost data. If you're lucky the virus scan log will tell you which files had the errors so you know what data you've lost.
If that number is stable then you may be okay (aside from the data loss), but my personal tendency would be to replace the drive anyway as Hawkeye suggests.
If the number is going up then your drive is deteriorating and you should immediately get a backup of everything you can and move it to a new drive.