I'm probably different than most, but you already know that.
For me, and for my wife, Lille is very much a love/hate relationship.
9 times out of 10, when we cross the Channel to visit Europe, we spend at least one night, either our first or our last (often both) either in Lille or within a very few km, often staying in Englos, a western suburb. We used to stay in northern near suburbs but after our two fav hotels changed hands and after a very unpleasant experience a few years ago at a B&B (also on the north side) we tried staying southeast by the airport in cheap and cheerful chain hotels, and switched to Englos several years ago. We are not the usual Rick Steves traveler - rather than going into the centre we tend to stay more like a sort of a regular short term temporary local - dealing with few attractions but doing the grocery shopping and stocking up on things I can't get at home; and getting dinner inexpensively but wholesomely at a fav restaurant chain.
So not very touristy. Again, the little bakery we go to for macaron is well out of the centre. I've never taken the bus there, we find (difficult and challenging but possible) street parking in our car from home.
We have always had reasonable access to the Métro in Lille so we have been into the centre and the old town several times, but to be honest it has never appealed to us. We find the pavements (sidewalks) in the centre very hard to walk on. They are not level, they are busy, and very very narrow, and heavy car traffic, and with my wife's vision issues it is pretty hard and dangerous.
The architecture is pretty nice in places, very modern in others. I understand that the museum is pretty good but never went in. Shop windows are pretty nice in certain parts of the old town but with the narrow pavements they are a bit close.
I understand that the fortress is pretty good, but we've seen so many similar in the Netherlands that we haven't developed an interest in this one yet.
The bakery chain Paul is from Lille, one of the originals is near the stations. General Charles de Gaulle was from Lille, and Le Creuset pots and pans aren't made very far away. The Field of the Cloth of Gold is pretty close too.
Both of the stations are easy to use, the Eurolille station is much more modern, functions well, and has the fairly large shopping centre attached (well across 50 m of open concrete, but basically attached except in awful weather, ask me how I know) which has a modest food court and two - count 'em, two - branches of Paul.
TGV train service between Lille Eurolille and Paris Gare du Nord is excellent and fast.
I know you aren't driving, but for those who might, driving in and around Lille is the pits. Low speed limits, lots of speed cameras, and if you can imagine crossing Belgian drivers with French, you get the worst of both worlds - awful lane control, too fast, drive right up your backside and are generally awful drivers. Belgians, by and large, have never found the turn signal stick on the steering wheel. They don't look behind before making their dash across all lanes either. The drivers in Lille have learned from them and adopted similar behaviour.
Belgium by the way abuts Lille to the north. We prefer to visit villages and towns on the Belgian side of the border. Less than an hour in the car to Ghent, by the way. My wife just came into the room and said, "We just don't like Lille. Lots of people do. We don't".
Never visited Christmas markets in France (Luxembourg, Bruges, yes, Germany, but never France) so can't help with that.