I'm currently up with both my investing and borrowing activity on BTC Jam. I think it's a great site.. However, that's not to say the site doesn't have its problems.
I actually put together a 'companion' site that scrapes all of the publicly-available data on 'Jam, and presents it in a more easy to read fashion. You can view it at
http://jam.sd.ai/ - Along with other members of the community, I've tried to identify a few metrics that show good borrowing activity, and as a result help to identify good investments. (BTC Jam itself doesn't display data very well, and it's hard to discover what you need to know.)
On the back of this, I've invested a total of over 7 BTC, of which 4.8 has been paid back with interest, and only 0.01 is overdue. That's pretty good going. I've also managed to leverage my reasonable reputation to borrow amounts of BTC I wouldn't normally be able to get my hands on, and have been paying back on time.
The thing is, BTC Jam is set up to encourage you to scatter small amounts around a large pool of borrowers, but I find it's better to spend time researching the individuals involved, and investing larger amounts with those.
There are some things that the admins could do that would benefit, however. As far as I can tell, they don't take much action before the loan becomes officially "in default", which I think is ~180 days after the last overdue payment. (They can correct me if I am wrong about this.) -- In my experience from running a business, 9 out of 10 customers with overdue payments will cough up if you phone them and ask them to. BTC Jam have verified numbers for all borrowers, so this is really something they should be doing.
As I understand it, they're currently 4 people but have just got some VC funding so I'm hoping this will grow, and they'll be putting someone in whose job is to do collections. I'm pretty sure that will help.
There are scammers on the site, no doubt. But if you actually spend a little time checking up on borrowers first, you can usually avoid them.