I'm a NetNewsWire user from waaay back, but I tried Gruml. Not just open it and play for a few minutes; I used it as my only feedreader for a week. Launches fast, looks a lot like NNW, but then it's all down hill from there.
In NNW, I can navigate through my feeds lightning-fast with the keyboard: left-arrow to get to the feed list, up- and down-arrow to navigate between the feeds, right-arrow to go to the article list, up- and down-arrow to move between articles, right-arrow or enter to open the article in a tab, left-arrow to go back to the feed list, up-and down-arrow… you get the idea. I can blast through a hundred articles in dozens of feeds as fast as I can hit the arrow keys.
But not in Gruml. First of all, the whole interface regularly bogs down while it refreshes a feed, which it apparently does *constantly*; you can't tell it to just check for new articles every hour, for example. And while you can navigate from feed to feed with the arrow keys - like NNW - when you move to a new feed, it stops to refresh the article list, making you wait until it's done to hit right-arrow to navigate the articles, or to even *see* the new articles. Oh, and about 10% of the time, it fails to refresh the article list, leaving it blank until you navigate to another feed and ten back - only to wait for it to refresh *again*. Multiply those delay by a few dozen feeds, and in my experience, it takes two to three times as long to blast through my feeds.
To make matters worse, many of Gruml's keyboard shortcuts don't require the Command key. Start typing when you think you're in a form field, and suddenly you're accidentally mangling your article and/or feed lists. Lotsa fun!
Oh, and while Gruml has some interesting (if somewhat unfinished) social media features, it doesn't support scripts. In NetNewsWire, I use the Readability2 script to reformat articles for easy reading, and other scripts scripts that send articles to Instapaper, EverNote, Tumblr, etc. I found those very hard to do without in Gruml.
In conclusion, Gruml could become a decent competitor to NetNewsWire, but it's really not there yet.