I would say depending on base, reserve goes 90% junior. There are about 10% or so (like myself) who intentionally bid reserve - especially if you are junior enough that you get a "crappy"* 4 day trip working days you need/want off. I prefer weekends off and would rather work reserve (again, living in base) to get that, especially if I can swing a few days where I am not called.

I've been here 2 years in one of the more junior bases (ORD EMJ) and a few months have been able to get a line but generally just bid reserve. I have gotten a line before and it is indeed, nice.

We do have something called "Composite" lines which are what they sound like. They are a mix of flying with reserve days sprinkled in. They days you have flying, you are NOT subject to reserve rules. I've done this before and it is also nice. Commuters have a love/hate because sometimes its nice to know which days you will be able to get home, but it sucks if scheduling puts a RSV day at the end of your sequence.

As far as base choice, you show up on day 1 and after attendance, they write up on the board what is open, you then bid based on age. What is open varies greatly. Past few classes have gotten all bases - ORD, New York, MIA and DFW. All ERJ, no CRJ. Most senior base now is DFW. However, with recent vacancy bids, people have generally gotten what they wanted. I had a friend who was one of the most junior people at the company at a time bid and get awarded the base he wanted (ORD, remember, one of the more junior base).

Any more questions feel free to ask.

* - what one person considers crappy, another may not.