I own both the Pentax LX and the MX. Both are excellent, professional grade machines; the MX preceded the LX and has available - if you can find them - a winder (not hard to find) a 5 fps motor drive (IIRC), a bulk film back, data backs - now out of date, interchangeable screens, depth of field preview, etc. It is a delightful camera to use. Note, however: If you are considering macro shooting the MX does not have mirror lockup, although there are some kludges out there.
On to the LX. Weather sealed, yes, although how many lenses are? The camera is getting old, whether or not repairs will be available in 10 years is hard to say. CLAs aren't cheap, but nor are they for other pro systems.
The camera is a fully professional system, comparable with the Nikon F3. Its center weighted TTL metering is incomparable, able to handle down to -6.5 EV, IIRC. Anecdotal reports speak of metered exposures of HOURS, my experience was about 20 minutes in moonlight; Vivitar 18mm @ f4 with 100 ASA film. Since the meter reads off the film during the exposure, changing light conditions mean nothing. Fireworks shots are a breeze! No need for a viewfinder blind.
One horrid omission: There is no auto-exposure lock. Grrrr. The aperture preferred AE works very well, however.
Flash sync at 1/75. Speeds over 1/75 are mechanical, so battery failure is not necessarily a catastrophe.
Mirror lockup, of course, interchangeable screens and finders, yes, if you can find them. Winder, motor drive, data backs (out of date!), yes. Excellent viewfinder, full info there. TTL flash metered off the film with the proper unit. TTL flash works very well.
And of course with both the MX and the LX thousands of K mount lenses, and the old 42mm screw mount glass (with an adapter and used manually) are out there.