Speaking of TWSBI, I'm giving one as a gift. It'll be the first time I'm giving a fountain pen to a non-fountain pen user, and I'll be tacking on a bottle of Lie de The and a Rhodia notepad. I only filled the pen with a bit of ink so that I can explain the inking process.
That is a pretty awesome gift! I hope that they absolutely love it

.
Bought a Pilot Falcon with an FA nib, and I dislike it. For a $100 pen it feels cheap. I find it hard to break away from my Lamy Studio (stainless steel). I find it hard to justify the cost. Any other suggestions? I'm looking for a pen with a bit of heft and with some flex in the nib.
How disappointing. sorry Treo360.
I am going to repeat some points that have already been made here but for what it's worth I would say:
Many fountain pens use plastic but it's good quality plastic. If it is purely a psychological barrier as 4Tran suggested then you might be able to overcome that by re-framing how you think about it. However, if the issue is also/or the heft of the pen then you are going to want a pen that is likely not made from that material. Goulet Pens have the weight of pens listed in the 'technical specs' tab e.g.
https://www.gouletpens.com/pilot-falcon-fountain-pen-red-soft-fine/p/PN71621 so that's a handy resource.
Regardless of all this however, if that's how you feel about it that's how you feel about it so there's no point trying to pretend otherwise. The question is really what you do about it. Do you try and get your head round it and come to like the pen or look for an alternative?
4Tran's suggestion of a Jinhao X450 or X750 is a good one. These take a standard size 6 nib and are easily swappable. There are several online text and video guides out there that demonstrate how easy it is to do. Some people put flexible nibs in them that people use for anime drawing. I have one of each of these on order from ebay that cost me £4 for the pair and that's what I'm going to do. For the sake of a nominal amount it would be a good way for you to ascertain whether your issue is the material used to make the pen, the heft, the nib flex or a combination thereof.
If this does not attract as an option however you might want to consider a pen with an antique nib as this is more likely to give you a decent flex than the majority of modern nibs. You might like to consider buying a new pen from a nib meister such as
http://www.fpnibs.com/en/39-nibs or
http://meisternibs.com/ with a custom ground nib or a replacement nib to your tastes.
Lastly, if you just want a straight suggestion of "buy this pen" because it's not made from plastic i.e. solid brass covered with lacquer so it has good heft to it then I commend this video from the wonderful Mr Brown to you:
https://www.youtube.com/watch?v=l7skrabcAlw I do not have this pen [yet] but it seems to have a fine recommendation online and it fits your criteria. At the end of the video he gives a nib demonstration so you can see the nib flex from that. The website for the pen is:
http://www.mrpen.co.uk/contents/en-uk/d184.html The pen nibs are custom ground and there are 18 different options! An alternative from the same company:
https://www.youtube.com/watch?v=drkVfgi530k
Good luck!