I have been a bluegrass fan all my life.  I always dreamed of learning to be a bluegrass picker and learning and instrument.  Recently my father bought a Dean banjo and started taking lessons.  This lit a fire under me to learn an instrument so we can pick together.  I decided to complement his music by learning to play the mandolin.  I spent days doing research only to find many conflicting opinions about what type of mandolin to buy to play bluegrass with.  Should I buy an F-Model or an A-model.  There were a lot of people who said that the F-model was the only one to play bluegrass with.  I guess because Bill Monroe invented bluegrass with a Gibson F-Model mandolin.  I heard a studio special on XM radio bluegrass junction with Ricky Skaggs.  Skaggs actually played with Bill Monroe and is a famous mandolin picker himself.  After hearing Skaggs talk about how worn and beat up Monroe's mandolin was I began to suspect that beauty wasn't as important as sound.  I found an ebay seller named musicsupplier who is an actual brick and mortar music store with a real phone number.  I recommend that you save yourself a lot of headaches and do business with someone like this.  I called them and after talking to Chad for half an hour and asking a million questions I found out that I could buy an A-model mandolin with a solid wood top (a better quality instrument) for the same money I was prepared to spend on an F-Model Mandolin with what turned out to be a laminated top.  Here are some things I learned that may be helpful.

First consider what type of music you want to play.  If you are into Celtic mandolin, then an A-model with an oval hole will probably be the direction you want to go in.  If you are into bluegrass picking, then sound is more important.  An A-Model or F-Model with f-style holes will have better projection and will sound great for bluegrass.  The only real difference between the two is the shape of the body of the instrument.  F-Models are fancier with the scroll shaped body and an A-Model is just an oval shaped body.  What is really important is the quality of workmanship and materials in manufacturing the instrument.  The solid wood top gives much better tone than the laminated top and will wear better over time. 

This leads to the second consideration, price.  Determine how much you are prepared to pay.  I had a budget of about $200.  In light of this I was willing to give up the fancier look if I didn't have to sacrifice sound and was able to get the most valuable beginner instrument for my money.  In general try to avoid the under $100 models.  They are generally of lesser quality and will not be as reliable keeping a tune etc. 

One thing to keep in mind.  If you purchase your instrument online, it will most likely come in the factory box which means you will either have to setup the instrument or have a music shop set it up for you.  Instructions for setting up the bridge and tuning the instrument can be easily downloaded from the internet.  Just do a search for setting up a mandolin bridge.  I did this and it only took about an hour and a half to set up my instrument and get it tuned.  I would recommend that you get a decent digital (chromatic) tuner as this made tuning the instrument much easier.  If you want to avoid this much work,  do business with a real music store seller and contact them before you bid to see if they will set up the instrument prior to shipping it. 

Don't believe people who tell you that you have to buy a $3,000 F-model mandolin to play bluegrass.  This is simply not true.  I bought a Kentucky KM-150S A-Model mandolin with f-style holes (which I was told do offer better sound projection for performances) and I am doing just great learning to play my favorite bluegrass tunes.  You can buy a great quality instrument for under $200 and do just fine with it. 

The moral of the story is do your homework, get a good quality instrument and have fun playing.  My mandolin has not had to be retuned in two weeks.  This is solid quality.  If you are in the market for a good quality instrument, I recommend the guys at musicsupplier.  They are first rate.  If you are still unsure if the mandolin is for you, I can tell you that it is very easy to learn the basics on.  I hope this helps.  Good luck and have fun.