I've been to Monsoon ID 1U few times, and that salon was my first (and only till now) hair salon experience.

I don't have a preferred hair stylist as I don't go there very often (like once in two months or so), furthermore, usually when I go there, the hair stylist from my previous haircut would be either not there (on leave) or transferred to other branches (due to the opening of more Monsoon ID outlets).
So I don't really have like a frequent stylist whom I go to.

Nevertheless, I had good experiences based on the times I went there.

The stylists were okay, managed to create the look I described. Usually I don't have any design or hair style I want, I just let them know roughly what I want or don't want; e.g. want to keep my fringe, don't want to keep long side burn, want to have shorter length and etc.
Then if for instance, if a too short fringe would look weird on the current hair style, the stylist would advise me not to shorten the fringe too much and say to keep a certain length.
I would say most of the times the stylists listened to my wants and needs well.
That again, depends on the hair stylist you get as different people have different etiquette and working style. And I have only worked with like 3 stylists before only

As for other services like shampoo'ing the hair, I didn't really face much problem here; as most would have enough experience to shampoo the clients' hair properly.
However, there was this once with a new shampoo boy, he was a bit rough and accidentally lightly scratched my forehead lol

The stylist also offer to style your hair after the hair cut, they used Sebastian Xtah Crude Clay if I'm not mistaken
If you're not satisfy with the cut, can just ask them to try to fix it. Then again, you might risk worsening the cut lol; as generally, you and your hair stylist might have VERY DIFFERENT views on what looks best on you.

The down side is the price. I think the price has increased after new year.
It used to be about RM33 for a hair cut + wash; but I just went there a couple weeks ago and got charged about RM40. They said it's a new pricing for new year.
So you might want to wait until they have promotion lol

In my opinion, usually most hair stylist (stylists from other salons as well) will be able to deliver the desired end results; it's just that we as the customer/consumer have to give enough explanations about what we want.
If clear instructions are given, the results won't sway too far from our description or too terrible. It's only too terrible or horrendous when no clear description and instruction are given; or when we allow the stylist to "terrorize" our hair without stopping them; or when the customer and stylist cannot communicate at all

Hmm, I think if a salon uses a certain product (say shampoo, styling products, etc.) chances are the product will be available for sale through them.

I think the last time I went to Monsoon ID my stylist did asked me if I need to purchase any styling product or not; but I don't style my hair so I couldn't be bothered
So I'm guessing the clay can be bought from them, I have no idea about the price they're selling though.

On the other hand, you can try getting the Sebastian clay from K Care Shop.
Based on the prices listed in their website:
50g - RM 48.00
125g - RM 90.00
I am unsure if these prices are reasonable or not though

You can try checking their 'Store Locator' to see if there's any of their outlets near your area. Then you can personally visit the salon to check it out

Here is the website:
*CLICK*

p/s: I am not affiliated with the K Care Shop and I actually have no experience buying anything from them; it's just that I so happened stumbled upon the shop's name when I was researching about the availability of the clay in local markets.