I replaced the FSU (the "Hedgehog") in our e39 last year.
Other than laying upside down in the passenger footwell, its not a bad job. It will certainly help if you have smaller hands. Access is fairly tight, and its tough to see/access/release the somewhat delicate tabs that hold the connector to the old FSU.
The other benefit is that the unit has been updated by BMW. The updated relay looks quite different from the original, and hopefully won't be susceptible to the same failure. Our auto climate control had gone nuts, with fan speeds randomly rising falling when the A/C was on. Has worked perfectly since the new updated FSU was installed. In my case, it also cured a mysterious battery drain as well.
There is a very helpful thread on the procedure
here.
Tim