Well, I know what it's like to post here and the frustration of then being criticized for something you are so proud of. I understand how frustrating that is, more than you know. I've often posted pictures here on lawnsite hoping to get a ton of praise, only to get more criticism than praise. That's incredibly discouraging and anti-climactic when you are so happy of what you've done.
So at the risk of being one of those discouraging people I'll give you my feedback.
First off, it's 300% better than it used to be. I mean, from a before-after perspective, it's amazing. No doubt about that. That jungle of a mess they had before looked horrible. And this looks nice. So you've done well with probably what budget they gave you.
Now on to the constructive criticism.....sorry......
Yah...I'll have to agree with a lot of the others. The waterfall is okay. But it really should look more natural. Even on a small budget, you can make a little more natural looking waterfall. Here's a small-budget one we did recently;
http://www.lewislandscape.com/photos/gallery/Water-Features/Dolman_Water_Feature_B
It's not my most favorite water feature we've done. And I wish they would have had the budget to put even more plantings around it then we did. But the thing is even though it's just a stack of rocks, we hid that fact fairly well with the planting we did around it. Again, I think we could have done even better. But the point is you shouldn't just stack rocks up and call it a water feature. Even on a small budget, making it look as natural as possible is an important goal. You may not quite arrive at that goal totally. But not trying at all is kind of bad.
If you are going to stack rocks to create a waterfall, like Waterscapes By Design said, you should definitely at least put a variety of larger plants and trees behind it to avoid the "Mt. Vesuvius Effect."
On to the flagstone patio...I like the ledgestone border. I always think that's a nice touch to flagstone patios. But what I don't like is the color of the flagstone at that property. We did a flagstone patio job recently using almost the same color of flagstone. But in that case, it matched the rock facade that was on the house and the other rocks that were in the landscape. In your case, those colors really clash with the house in particular. The shape of the patio and walkways is suspect too. But I won't be too hard on you for that. I'll leave that alone for now. It's not too awfully bad.
The other thing that really bugs me is the location and color of the iron Arbor / Trellis. Again, I don't think black helps the overall look of the back yard. I think the yard needs brighter colors. A white one would have complimented the house and brightened up the dark landscape better. But the location kind of bugs me too. These Arbor / Trellis things should really designate a point of entry or a change from one area to another or a final destination. For instance, they are great over gates because they help distinguish the front yard from the back yard. They are also nice as a final destination, in the corner of a landscape, with a bench under them. Or they are good for when you are changing from one kind of hardscape to another. And you'd usually want to incorporate a gate then too.
I also agree with the more planting comment. Although I totally understand having a customer who just doesn't have any more room in their budget. I've done lots of jobs like that where I only WISH I could have planted it like I had wanted to but they just couldn't afford it. Still, it would have been a lot nicer with some more mature plantings.
Not a bad job overall. Just trying to offer some constructive criticism. That's how we all learn. I am still very much learning on every project we do.