Summer is low season in Thailand so you can get good deals on hotels. It is rainy season, but it's not bad to the point where it will interrupt your trip. Sep-Oct is peak rainy season.
FYI, Thailand is reinstating the TM6 form starting in May, but it will be an online form instead of the paper cards they used to hand out on flights. You can fill it out startring 72 hrs before arrival. Also, visa free stays are being shortened to 30 days instead of 60, but you can always do a 30 day extension at the local immigration office.