Hmm what a can of worms your question could open up.
Negatives:
The biggest one is can you trust him to look out for your best interest. Brokers work on commission. Work with someone who will disclose exactly what they are being paid and will reveal the true lowest price and let you decide how they get they paid.
Points good or bad. For most investors bad just because you probably will not hold the property long enough to recover that money. Not always but most times.
Positives:
Access to many different lending programs. Usually a lot more options than the local bank.
Flexibility in the fees they charge. Negotiate
Willingness to discuss lower fees for repeat business and referrals
Just a few of the negs and positives. Add a few more folks.
Be careful with brokers. I applied for a refi on a townhome with a local attorney's office who had a good reputation. The the actual loan was processed in Florida and up the line I found out the company only had 3 vendors they used and wound up shopping my loan on an open call system.
I got 3 inquiries on my credit and a loan officer that couldn't read investor tax returns. Tried to get them to take those inquiries off of my credit bureaus and they wouldn't budge. I've never had a problem with bankers or anyone else locally. Just tracking a referral from a friend. Also, had a salaried job.
here is how i deal with inquires on my credit files.
http://www.idtheftcenter.org/Factsheet124.pdf
before i got the credit freezes my scores were a solid 750's. since i put these freezes on, my scores are now low 800's. my broker swears the reason is because no one can access my credit.