As with everything, there are good and bad relo companies out there - unfortunately, the company assigned to deal with our move definitely fell into the latter category.
Cost - IIRC, 10K+. (This was eight years ago) The fee was tied to the monthly rental, plus fees. Seemed quite excessive, given that they were only able to show me a few properties...
Service - Unorganized and unprofessional. I had limited time available for a house hunting trip, and expected to spend my three days here efficiently. Had several conversations with the firm outlining my expectations, etc. I arrived in Zürich only to find they had not booked any viewings - and I certainly didn't appreciate their suggestion that I might want to spend my time shopping on Bahnhofstrasse instead.
I ended up having to schedule a second house hunting trip later.
Secondly, the firm kept pushing properties that were significantly more expensive than we had budgeted. I got the impression that they were trying to milk a corporate move for all they could.
Once we chose a house, the relo firm took over the contract negotiation and handover (that's their job, afterall.) Mistake. The firm signed a five year lease with a huge penalty for early move out. Given that they knew this was a one, max two year assignment - pretty ridiculous.
They also did not ensure that repairs for damages caused by the previous tenants were done, as had been promised.
When we later moved out, I found out that the relo firm had not done the handover protocol. (We didn't even know this was a necessity) We ended up having to fight charges for the previous tenant's damages.
So... if you have a choice of relo agents, make sure that you check references and ask about others' experience with the firm. It also helps to have a little understanding of the Zürich housing market so that you can tell when your are being fed a line...
In all fairness, I arrived in Zürich speaking no German, knowing nothing of the how the rental market worked here, with a tight deadline. I doubt I could have found a place on my own.