I've run 51.5 with 51. You can read a detailed report in my campaign summary thread in this forum.
In sum, I ran 51.5 after they had done levels 1-2 of Castle Whitrock. This meant I had to boost a few things here and there, but not too much.
As an introduction to CW, I think 51.5 is a pretty tough adventure for first level characters. If they do make it to the end, you can expect them to be 2nd or maybe even 3rd level, which means they should have a relatively easy time with the first level, anyway, of CW. This could lead into a situation where they are always roughly 1 level higher than that needed for the level they are about to explore (a situation which my current group is in, as they are 6th level and getting ready to explore level 4).
Positives: 51.5 is a lot of fun, with interesting level design and fun traps and situations to puzzle out.
Negatives: A few things in 51.5 could have used improvement, and these are matters you should consider before running it, in case they come to the attention of your PCs. These include:
1. How do the sih'hel get from the bottom of the level to the top? The elevator room seems to be a one-way thing (see below), and it is unlikely they go through the owlbear lair, which makes accessing the surface difficult. I decided that they could exude a sticky substance from their fingerpads to allow them to climb the stone walls, or at least give them a good bonus on climb checks. The massive overhand near the bottom explains their need for the rope ladder.
2. The owlbear is effectively trapped in its room, with no way out. I ended up keeping it there. I figured the sih'hel trapped it down there, and it lives on rats and the occasional carcass or prisoner tossed down into the well room.
3. How is it possible to navigate the mine cart passage to CW, behind the big door? According to the description it is 2 feet square, which is too small for Medium creatures to squeeze into. I simply enlarged it to 2-1/2 feet.
4. The elevator room--can it be raised? Does it raise up on its own? Do the sih'hel ever use it? I put a lever at the bottom to allow it to go up, and decided that the sih'hel have used it before, but stopped after it started making all the noise.
As I recall, there's no duration listed for the effects of the squealing noise. I believe I made it last for the duration of the sound, plus 1d4 rounds thereafter.
5. The crushing walls trap in the secret room--this may have just been me, but I felt the portcullis/closing walls thing felt too easy to escape. You might consider increasing the depth of the chamber, to force characters to run FAST to try to make it out in time.
All of the above are fairly minor issues, but worth noting! One final thing--the sih'hel chill touch thing requires them to grapple, and they have TERRIBLE grapple scores. If you want to toughen them up a bit, you might make this a touch attack effect, or give them bonuses to grapple checks. I didn't do this, and the sih'hel were pretty easy for my group to kill off, even when I started adding class levels to them.
Author: Mother of All Encounter Tables; Bits of Magicka series
Coauthor: Rappan Athuk Reloaded
Castle Whiterock Kills: x10