Depends on what you mean by "completely screwed up". You can always reflash if the problem is in root filesystem, initfs partition or linux kernel. (/dev/mtd4 3 or 2) You can really brick the device if you mess with bootloader or config partition (/dev/mtd0 or mtd1). 99.9% problems are because of something broken in rootfs.

Reflash will fix it but you can also install bootmenu (and optionally clone your system to card) before you start experimenting with your system. Then once this happens you can either boot into usb network recovery mode, ssh in over usb, mount rootfs and fix it or you can boot device with copy of clean system and mount and fix broken rootfs too. All this without reflashing and losing your data.