Brand new 735XT

There's nothing wrong with buying old models, if the price is right.

For Black Friday Garmin is practically giving them way.

There is nothing wrong with old tech.  It will miss nice features, and it is hard to imagine the battery enjoyed sitting in storage for a couple of years. 

But the price is right.

Just purchased one at the Garmin Black Friday sale.  The battery was completely dead but it charged back up and seems to be holding the charge.  It was running Version 7.10 (which I updated to the current version 9.80).   I compared the HR with my Instinct and they seem to be very close.   The 735XT features are a little different  but it seems easier to find them and it's a lot lighter and streamlined.  I'll use the Instinct when I'm out crashing around the woods. I actually think I might like the Forerunner better for training and everyday.  SO far so good.

After a couple of weeks, does the battery seem to be working normally? 

Yes.  The battery seems to hold a charge just fine.  I let it run completely down twice in case it was that kind of battery.  I turned the watch off for 10 days and it still had the same charge when I turned it back on.  I then ran it for 3 more days before it needed another charge.  I really like this watch.  Still learning how to use all of the features but already like it better than my Instinct.  It has a lot more features and more suited for the training I do.  I ofund an app that will help me pace my next marathon in the IQ store.  Peters Race Pacer.  Actually I love this watch.  The battery is lasting longer than the Instinct and a lot easier to plug into the charger.  Thanks Garmin for putting it on sale!!