Not personal experience, but based on users' feedbacks. And when I tried it at ehome, they recommend using EXT3 because most 1080p movies might stutter.
I use an HDX 1000 NMT - there is no problem at all with external drives formatted in NTFS.
I have NEVER encountered stuttering with any 1080p material. Even large 30-40GB files play seamlessly. Similarly, the Popcorn Hour A-110 does not stutter when playing 1080p material from an NTFS drive. (Chances are, one day, I will encounter a file that stutters - but this would be an isolated issue, perhaps due to the encoding method)
As far as the HDX 1000 is concerned, I recommend EXT3 file system for the ff. reasons:
1. EXT3 provides
both Read/Write access from the on-screen menu, as well as over the network
2. Fast response when navigating the on-screen menu (especially when changing media sources)
NMT devices are designed to read NTFS drives; if you are encountering difficulty with 1080p material, it may be a problem with the design/hardware implementation for that particular device. You may also want to check your USB port and external hard drive to see if they're performing per specs.