Last year, we celebrated Christmas with the neighbours. It was really nice.
It's still nice today that we did that last year, but the message I want to convey is that is was a nice thing at the time we did it.
She gave me a new computer for my birthday. It was really nice.
She gave me a new computer for my birthday. It is really nice.
Here, the difference is indeed that in the first case, I either mean that her giving it to me was nice, or that I no longer have that computer (or it is not nice anymore — possibly because it is now out-dated).
The second sentence very clearly states the computer is nice, and it still continues to be nice.
Now, about having seen an item in a store.
If I'm out shopping for a sweater, and after having seen some, I am talking about the one in the first store we went (of course!), I'd say:
The sweater in that first store we went... it's really nice.
Meaning that I still think it is nice, and basically implying that I might want to go and buy it.
However, if I have no intention of actually making the item a part of my present life, then having seen it, as well as appreciating it, is something that occurs in the past:
That sweater in the first store was really nice, but it was too expensive.
Sure, chances are that it still is nice, and still too expensive, but that is not relevant to my current situation.
Compare the following sentences:
The girl I met yesterday was really nice.
The girl I met yesterday is really nice.
The first sentence implies that meeting her is something that happened in the past and that is now finished. I have no idea whether she has changed much since then, but it seems reasonable to assume she is still nice. However, since I have no plans of meeting her again, and since she is not part of my present life, it is quite irrelevant whether I consider her a nice person in the present.
The second sentence does drag her into my present. I think she is nice, meaning that I think of her as being in my life in one way or another at present. I am probably planning on meeting her more often.
My colleague at my current job is a really nice person.
My colleague at my last job was not very nice.
Again, teh first colleague is part of my present, so the fact they are nice is part of my present. The second colleague, as well as their not being so nice, is part of my past.