Hi! There are some great answers here already, but I'll chime in with my best attempt at an honest, unbiased answer too.
MATERIALS
The leathers RM uses are fantastic. Some of the best leather I've ever seen on a handbag. You can tell she puts a huge amount of effort into choosing her leathers. It is all very high quality, touchable, and wonderful. The RM leather is what first drew me to the bags and I really appreciate that, unlike some other companies (ahem, ahem, Botkier!), she has not started to skimp on this as she has had to produce more bags to meet demand. She also uses a wide variety of types of leather so there is something for everyone. I really can't say enough about the leather. It is amazing and she has wonderful taste. Ditto the colors. She picks great colors.
Some of the other materials have issues (zippers, hardware etc.) but I will discuss that in the construction area. Overall though I give Materials an "A" with an "A-plus" for the leather!
DESIGN
The design of the bags are also totally fabulous. This is my SECOND most favorite thing about RM bags, after the gorgeous leather. Her designs are totally adorable and extremely functional. You will become addicted to the pockets. I give her designs an "A!"
CONSTRUCTION
Construction is the area in which RM bags do have some issues. The handles issue, mentioned previously, is a big one. I was one of the first people on the board here to have an RM bag - I purchased my first over a year ago. I LOVED it for six months (still do!) and carried it a lot, but gently. Within six months, the handles had worn through at the bottom, bent, no longer held their shape, the outside leather had worn partway through and the stuffing inside had come apart. I have not carried this bag for months because of this.
This was a disappointment, but for a bag that I got for $375-ish (the price of an RM bag at the time!), and used a lot for that amount of time, it's not a big deal. However, if you are going to charge $600 for a bag...this should not happen. I am not the only person this has happened to and I am in no way hard on my bags. I have other bags with rolled handles that I have had three times as long and have carried much more that still look new. If you press lightly on the handles of an RM bag, you can feel how flimsy they are, esp. at the place where they connect to the bag. It is a construction problem. However, the wearing through seems to happen more to certain styles of bag than others and maybe only certain leathers (like the large size MA in royal with basketweave was mine with the problem, whereas I have others that are fine). It has not happened on any of my newer RM bags yet but the handles look and feel exactly the same, so I am wary and find myself not carrying the bags as much. I have noticed it is mostly a problem on the regular size MA in certain leathers - I suspect that bag might be too heavy to be supported by these handles.
In addition to the handles issue, members here (though not myself) have experienced hardware falling off, hardware tarnishing (this is more an issue on the old hardware; I think she's fixed it with the new hardware!), nameplates falling off or partway off (this actually did happen to me!), etc.
In summary, I think that RM started out as a lower-end brand than she is positioning herself as now. On the positive side, she is aware of a lot of these issues and is working to correct them -- she's great with this; the new hardware is much sturdier than the old. She has also acknowledged the handles problem and says it is fixed. However, the issue of construction is quite honestly the main thing that is holding me back, currently, from purchasing these bags at full retail price of $600 and more. I am waiting to see the fall line to make sure all these issues are addressed. I am not a girl that has 30 bags that I rotate daily -- I want a bag I can carry every day or almost every day, treat nicely and normally, and expect to be sturdy. Still waiting and hoping that RM bags will stand that test in the future, because everything else about them is so great.
OVERALL QUALITY
Great leather, great design, offset by some lingering construction issues that I hope are fixed in the coming season. I give the overall quality a B to B-plus for the price (which is now $600-ish).
CUSTOMER SERVICE
RM's people really could not be nicer and more accommodating. As people I give them all "A's".

However, her company is growing so fast and it seems like she does not have enough staff. When you order something from her website it can take up to a month for it to ship to you, and sometimes you will get no tracking. I now go mostly with boutiques instead because they always ship much faster with tracking and have fewer problems.
However, RM and her staff are always really awesome answering questions here and being attentive to us on this board. That is really unique. So overall I give Customer Service a "B-plus" and am sure it will be "A" level when she gets more staff.
Hope this helps
