What are your recommendations for SDXC cards for the FS5? Are there any you've had problems with?

You're obviously in the US, but in the UK I would recommend this by Sandisk:

SanDisk Extreme 64 GB SDXC Class 10 Memory Card up to 90 Mbps with U3 Ratings, Frustration Free Packaging: Amazon.co.uk: Computers & Accessories (http://www.amazon.co.uk/gp/product/B01469QK0Y?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00)

It's as fast as you can currently get (U3 class 10), is supplied by Amazon themselves (so not fake), has excellent customer feedback and isn't too big. Remember the FS5 can only write 200 clips to any one card, so unless you record quite long clips anything over 64gb will probably be pointless.

So far I've had no issues with the two Sandisk cards I've purchased, but to be fair I've only used them on a handful of jobs.

Here's the US version: http://www.amazon.com/SanDisk-Extreme-Memory-SDSDXNE-064G-GNCIN-Version/dp/B013CP3MMM/ref=sr_1_4?s=books&ie=UTF8&qid=1457903498&sr=8-4&keywords=Sandisk+class+10+sdxc+64gb

..and a few dollars more for the Pro version: http://www.amazon.com/SanDisk-Extreme-SDSDXPA-064G-X46-Flash-Memory/dp/B007NDL54C/ref=cm_cr_arp_d_product_sims?ie=UTF8

I'm using these PNY SDXC 64's as my daily drivers:
Amazon.com: PNY Elite Performance 64 GB High Speed SDXC Class 10 UHS-I, U3 up to 95 MB/Sec Flash Card (P-SDX64U395-GE): Computers & Accessories (http://smile.amazon.com/PNY-Elite-Performance-Speed-P-SDX64U395-GE/dp/B00WWBCSG4/)

I keep a 256GB of the same type in the B-slot. I dual record. I use only the 64's to shoot and edit, the 256 stays in the camera until it's full and then is swapped out with another. The 256 is only backup - the goal is that it never gets removed from the camera or read in a card reader.

Remember the FS5 can only write 200 clips to any one card, so unless you record quite long clips anything over 64gb will probably be pointless.
I'm looking at a 256GB card with 500 clips on it right now. 500 is an oddly round number for a random number of clips, so it's possible Catalyst Browse will only recognize 500 files (which would be equally odd), but there doesn't seem to be any problem recording or playing back these 500.

Mike is probably right that it's 500. I recall it was a round number from Doug Jensen's Master Class videos. But for some reason I thought it was 200. I remember Doug recommended 64Gb cards though for the reason that your average shooter would hit the clip limit before the card size.

i buy 2 this Lexar Professional 128GB Speicherkarte (1000x Speed SDXC UHS-II)

I've been using these on my x70 with no problems.
Transcend 128GB Ultimate SDXC UHS Ultra High Speed Class 3 Memory Card https://www.amazon.co.uk/dp/B00J3KA8JG/ref=cm_sw_r_other_awd_U4Q5wbC2VXY9P

From the manual:

• The maximum number of scenes that can be
recorded is 600 in XAVC HD/QFHD and 3,999
in AVCHD.

• The continuous recording time using relay
recording is 6 hours for XAVC HD/QFHD and
13 hours for AVCHD.