First off, this isn't in Philadelphia, it's in Conshohocken. I don't know why it's listed as Philadelphia.
So I was driving up Fayette around 11:30am, looking for a deli I went to years ago. My partner and I found it, but saw the 401 Diner kitty-corner and thought I'd try it. It doesn't look like much from the outside -- kind of a silver diner look with an old, faded art deco sign. I expected an old diner inside too, but it was totally different. Light, interestingly tiled tables and coat hooks, little colorful wooden flowers ($8 at the register) and other decor. Lots of wood if you're into that sort of thing (and I am). The average age of the patrons was about 25, with one young family and one older couple.
I just read through the other reviews and my experience wasn't awful at all. There was an early error: no silverware for my partner. And his coffee was incredibly hot and strong. But that was it for the bad bits. I asked the waitress whether to choose the salmon burger ($12) or turkey burger ($11), and she enthusiastically suggested the turkey -- plus "truffle fries". Our order was taken quickly and delivered promptly. He had chocolate-chip pancakes ($7.50) and I had the turkey burger. The pancakes were airy and tasty (yes, I stole one), and my turkey burger had a nice spicy sauce and sauteed onions. (And a fat slice of purple onion, which really overpowered things and I took it off.)
And the truffle fries! The waitress said they were drizzled in mushroom oil and sprinkled with cheese. They were awesome! They had a lot of flavor, weren't over-salty and the cheese added just enough punch. (My partner thought the fries really smelled too much, but the fries were all about me!) We boxed some to go, and the box was an odd stiff shiny compostable cardboard which I appreciated -- no foam!
All in all, I'd recommend it. The menu wasn't huge but it had a clever mix that should please everyone. It was a hair pricey for a diner, but not out of sight. It was interesting for all senses, though, and that made the experience a really fine lunch.
Warning: A sign on the door today (March 30, 2013) said that they would be "temporarily closed" for dinner, closing at 3:00pm for the foreseeable future. Don't know what that's about.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.