Being a PS3 owner I can only judge Valve on the few games of theirs that I have played and My verdict is: pretty good. Keep in mind Valve hates my chosen console. I played portal and the orange box and loved it. I have a few 360 owning friends and when I have access to their consoles I cannot put down Left 4 Dead 2 (or Gears, or Vesperia but I'll stay on topic). Sorry to you L4D fans but I didn't jump on the L4D train till the 2nd game was released and I have to say it would be a different game if Ellis was not in it. In one moment your adrenaline is pumping, you're anxious and maybe a little scared then Ellis will randomly say something hilariously stupid that makes you laugh and breaks the tension. "This one time my buddy Keith [insert Ellis trademark punchline]", sorry but that added greatly to my experience. Also another apology to you fans of Valve, I do not find the cult classic series Half-Life to be entertaining. I am an avid gamer so I respect Half-Life in all its glory for what it is but it just isn't my brand of whiskey.
With all that being said, I think Valve is pretty good. I think we have learned that for the most part, and I will repeat for emphasis, 'for the most part', games that take a while to come out are worth the wait (let us not think of exceptions like FF13 for the moment) so I've got no problem waiting for Portal 2. Also, what is the big deal about engines? Do you realize how many games run off of the same engine even if they seem newer or better than their previous iteration? Do you even know just how many games are using the Havoc and Unreal engines?
First of all, a software engine is usually very versatile so it can be used in may different ways and what kind of software is used only a part of what makes a great game. The most important thing about a game is its overall presentation; it is all about how well it performs with what it is working with. We all keep crying for better graphics and new engines but I challenge you to find just one gamer who does not still enjoy classic games. Heck, that's why Nintendo is still making money. Also, crying for new software is dangerous talk. If those types of cries were always answered by developers then you would be crying "games shouldn't cost so much." If you noticed, developers have a strong since of high financial entitlement, which is not unreasonable. So if they deliver more, they believe they're paychecks should be higher we the gamers should be willing to gratefully pay more. I know have stated more than you have asked yet I just wanted to share a few thoughts.