How serious are blood clots in the legs or thighs?

Very serious. A blood clot in the leg has the potential to break free and float up to the right side of the heart, and into one of the lungs. This is potentially fatal. The risk is a lot higher if the clot is in the deep veins of the thigh, and less if it is in the calf. Superficial vein clots are not dangerous. If you have a clot in the deep vein of the thigh, you'll need blood thinning drugs like coumadin (warfarin).

Related Questions

How to know if you have blood clots in your leg?

Ultrasound. Venous ultrasound is diagnostic for blood clot in your leg.

What do you do if you have blood clots in the leg?

Get help Immediately. This is a dangerous situation which needs immediate care. You will need bed rest, Heparin followed up with oral anti-coagulant medications. How do you know you have blood clots? Was it diagnosed by a physician?
Blood thiners. If this blood clot is is a deep vein, treatment would consist of anticoagulation with 3-6 months of blood thinners. I would also strongly recommend the use of compression stockings to prevent future problems. A physician also needs to try to determine if this was a "provoked" or "unprovoked" dvt. If there was no obvious cause then further work up maybe needed.

I'm wondering what it feels like when you have blood clots in your leg?

Pain and swelling. Usually you will feel leg swelling and/or pain (could be very painful). Rarely no symptoms or just swelling.