Here's what I think about each of the four wonders in the Wonder Pack (from a game design perspective):
1. I find The Great Wall with its thematically fitting flexibility particularly interesting. A and B side seem nicely balanced and offer a real choice which side you want to pick depending on circumstances (if you play that you can pick a side as I do, that is). The 4th stage of side B enables a rather unusual and therefore interesting very low resource strategy (further enabled by the money from the 1st stage).
My rating for this wonder alone would be a 9.5.
2. I also really like the A side of Abu Simbel which lets you view some leaders in a totally different light.
However, I find the B side considerably too weak: It is relatively easy to have *one* non-VP leader which you can mummify nicely on side A at the end of age III after he has provided most of his benefits (or Pericles which is expensive enough to often be worth mummifying in spite of the lost VP). But it is much less probable to have two expensive non-VP leaders and even if you do, you'd either have to mummify one of him in age II already and thus usually loose many of his benefits during the game or you have to mummify both leaders in age III which can easily mean to dispense with at least one valuable age III card. (It happens often enough that you get one dud hand in age III with which you can nicely build a wonder stage, but two unusable hands are much less common.)
My rating for the A side would be a 9, but the B side would only get a 4. I'd combine this to a 7 for Abu Simbel overall (at least when letting the players choose a side).
3. I like Manneken Pis when playing with the popular rule that each player can choose the side of his randomly determined wonder at the very beginning of the game (like I do). I don't like the wonder if you can be forced to play the A side by drawing it, because depending on the neighbours it can be pretty unfavourable. Not only for you, but also for the neighbours, for instance if you (have to) copy science from Babylon or military from Rhodes or Catan B, thus luring both of you into strategies that usually suffer from neighbours who play it. But if Manneken Pis has the alternative to choose side B (which is quite a strong side I think), I like the whole wonder including the A side which is then an oddity whose strength varies wildly depending on the combination of neighbours, but has side B as a "safety net". Which means that the whole wonder seems really strong overall, but not yet too strong I think.
In any case I like this version considerably better than the previous promo version (see my comment for it for why).
I'd rate it with an 8 overall (when letting the players choose a side).
4. I hate the published Stonehenge, it is clearly too weak. I am convinced that it is the weakest published wonder board, at least the one for which it is most obvious.
This becomes particularly clear for its A side when you compare it with Gizah A. I simply can't see any way arguing around that, although I really looked and wanted to find something to redeem Stonehenge. But there just doesn't seem to be any meaningful enough advantage compared to Gizah A to set off the disadvantages that you need considerably more resources to make it score about as well (by collecting 3 or 4 Stones on brown cards for 14 or 16 VP with all three stages) and the risk of just not getting your hands on enough Stones.
Stonehenge's B side is considerably too weak as well judging by my experience of how many points you can expect on average from the second stage - not enough considering how particularly raw material hungry the B side is. You also have to keep in mind that it is much harder to use the stage as bailout for a weak hand as one would usually want, one has to have the right color card on that dud hand as well. Dud hands become much more probable at the end of the age with few cards in hand, but few cards in hand also mean that having the right color among them becomes much less likely.
My rating for the official Stonehenge would be a very disappointing 2.5.
Which is why I feel the need to house rule it, although I usually try to avoid that - but the official version just feels too unfair for the unlucky guy who draws Stonehenge otherwise. I have posted the variant I have currently decided for here
Stonehenge is the main reason that I rate the entire Wonder Pack 2.5 points lower than the base game.