Well the benefits as noted are fairly obvious. No amount of emailing is going to tell you whether you're going to click with your future boss and your working relationship with your boss is going to be crucial to the success of your stay here. Also to get a "feel" of a place yourself is not possible unless you can come and have a look yourself. A couple of minutes talking on the phone is not going to be enough to give you a fully accurate picture. Another important advantage with turning up is that you can talk freely to the teachers already there. Why not email or phone you say? Well if you ask for email addresses they will usu at best just give you ONE for various reasons. The teachers can get sick of answering emails about the school, telephoning can be a problem because people are always out. Also they can be reluctant to give emails because often teachers are depressed about this and that and might not give a positive testimonal. But if you just bole up they can't exactly turn you away and you can get to the bottom of things.
What about the risks? The risks of it not working out can cost a lot financially and be very high emotionally. Korea can be a very challenging place and if you're not in a good environment you can easily become stressed and miserable etc and end up having to leave. Still if you have a good long talk to the existing teachers before signing you should be able to get some idea and you can find out whether they pay fully and on time etc so you can to minimize the risks of getting seriously ripped off financially to a low level. A lot of people don't even try to email (which can easily be faked) little own phone the existing teachers but thats their mistake unfortunately.
Still if its not going to cost you much you might as well come over here first mightn't you since they pay airfare back don't they? Actually they won't pay your airfare back if you're already here. Instead they will just pay for you to do a visa run to Japan so thats going on a thousand dollars you will be out of pocket. But thats just the beginning. To stay here in Korea in motels for three weeks or so and traveling around looking for a good (remember thats why you came over here not just to accept the first offer you get) job is going to cost quite a lot. You can easily spend US$330/wk with motels, taxi fares and so on and it could easily take three to 6 weeks to find a place. Why so long? Well even if you locate a place within say 2 weeks you may have to wait another 2-3 weeks before the position is vacant. So you could be up for another $1.5-2000. So all up you you could be looking at an extra cost of US$2.5-3000! Wow thats a lot!
Is it worth it? Well even a year ago I would have suggested perhaps yes. There were just too many dodgy hagwans around and not enough reasonably honest ones for it to be worth the risk. With the currency crisis they were feeling the pinch and people were getting ripped off big time, left right and center. But things have improved. So many teachers fled from this place (often to Japan) and it seems like the school owners have lifted their game. Complaints that you used to hear often like having to do lots of extra overtime at no extra pay you hardly hear at all now. For those who do a lot of free lancing the lying and cheating is still noticeably common (practiced by the majority) but its mostly over small time stuff. Lots still don't easily give release letters. Some still refuse severence pay. But the places that screw you every way from Sunday are pretty infrequent now. I would make an estimate that "only" one in seven are now of this type. Theres still a risk but you're better off keeping your money as insurance than losing $2.5-3000 for certain. That way you can easily change jobs if they don't pay you or even go to Taiwan if things don't work out.