LockInfo
While I've never actually tried ISX, LockInfo already does everything I want it to do. I've seen screenshots of ISX here and there and it doesn't seem to do anything new that I'd want. I don't really care for RSS feeds, Facebook, or Twitter enough to be ready to glance at at any moments notice. I'm okay with unlocking my device and looking at their respective apps (Flipboard, Facebook, & TweetBot).
My most used feature of LI would probably be mail previews and it works great from wherever I am, wether it be from the lockscreen or within another app. I like that I'm also able to mark the email unread/read and delete the email all from withtin the preview. Since I haven't used ISX, I'm not sure if these features are included but like I said, LI already does it and it works.
Some other features I like include integration with the Calendar, Reminders, & biteSMS. I also use the repeating alerts feature within LI. It's more customizable and also allows me to repeat calendar alerts & reminders. Push alerts from 3rd party apps are also well integrated allowing for Tap to Launch and an option to clear badges upon dismissal of the notification.
These are just some of the features that keep me from switching from LI. Again, I've never tried ISX but LockInfo works great for me already.
One more thing, the developer of LockInfo, David Ashman, seems to be very involved with his userbase and does a great job of communicating. He's never failed to respond to any of my support requests and bug reports.