So far AdultDVDEmpire seems to bring the best result for me:
1st off, the selection of inventory including new releases is enormously satisfactory. i've found multiple titles that i just couldn't find among the other DVD online rental sites. Thus i didnt have near the long wait as i have had to endure with the others.
(2) Customer service seems quick to help and solve problems, the live chat help page probably brings the quickest results.
(3) i don't have the aggravating problem of waiting seemingly forever on new releases as i have experienced with other DVD rental sites. With the others i have had to wait months before a new release title comes in on my queue/rental list.
(4) the only problem has been the longer than desired turnaround in receiving/shipping of rental DVDs after the returns have been dropped in mail. It can sometimes come to at least 7 days. They may blame the post office, but i believe the receive/shipping process. And you have to be paying attention to the mailer envelopes. It's different from others, particulary the mailing address stickers on the front. If you don't remove them before you dropped them back in the mailbox, the post office will be re-delivering that same DVD back to your home again.

I've found Adultdvdempire.com to be the best rental site on the internet, and i've tried most of them. The selection is more than double what sites like wantedlist or sugardvd have. They have over 57,000 titles and they have lots of new releases available, which is very important to me. They add over a hundred new movies each week. I've been a members for a few years and the rental mailers they use have changed to an excellent, easy to open design. They also started including padding with the DVD's to reduce the number of items that get damaged by the US postal service. The customer service has always been prompt and friendly. There's a reason this company has one the AVN award for the best adult retailer for the past few years.

If you want fast turnaround time this is not the place for you.  I was renting two at a time and returning them as soon as I got them, and still only received an average of only 6.6 per month.  I’m pretty sure they sit on them for a couple of days when they get them and blame the post office.  I finally quit after I caught them claiming to receive only one of two DVDs that I put in the same envelope, and they still did not send the next one after I complained about it. 

Also, their customer review feature does not work.  I got ripped off when I ordered a “50 hour” compilation called Girl Talk that only contained 20 hours.  I wrote a bad review that still hasn’t shown up on their website three months later.  Meanwhile it continues to be one of their top sellers.

For Adult DVD rental program; they send videos from PA, so if you live in the midwest expect 2-3 days for delivery and another 2-3 days to return. 7-10 days between receiving the next video makes for a long wait. Also, many titles will be "out to customer" so it is likely that you can't get the video at top of rental list.

  Huge, huge selection. Their site is straight forward with lots of different search options to find what you are looking for. Like TheKana said, turn around time seems slower than it should be. That's the only down side.

Some good, some bad. I have used this along with 5 other services, and AdultDVDEmpire has by far the best selection; if they don't have it, odds are no one does. Rental plans are reasonable (especially the 8-at-a-time) and they give you a 10% discount after two months. Good customer service. They also have (for some titles) screenshots so you can make selections based on more than the front and back cover. The bad news is, they DO seem to hold returns to slow down the number of titles you get per month. They never seem to get around to posting user reviews. The website is a bit cumbersome and they do not have good preview pictures. They do have an inordinately high number of defective discs. Fortunately, their customer service is usually prompt and solves problems.

I'm using the first avail. option, and unfortunatly, they close their eyes and put their finger on the one they will send. I'm in NY, it takes about 2 days after they ship for me to get the DVD's (and the same with returning). Other than that it's ok.

Other than slow shipping their customer service is great. They understand that movies can get lost in the mail and they will work with you when this occurs. The only bad thing is that I believe that they delay shipping/receiving items to limit the number of movies that you can rent in a month.