I have a super pentax me with 50mm/1.7 and 40mm/2.8. I have seen some opportunities to buy SLRs from other systems such as nikon fe /el2/nikkormat ftn, rolleiflex sl35 e, canon ftn... But I was wondering if it was really worth it even if my camera has a lower value.
No, not worth it -- you'd be better off keeping the ME Super and your lenses (especially that 40/2.8, those are worth some dough!) and investing in the Pentax ecosystem.
I started out as a Pentaxian and now own several brands of 35mm SLRs, including the Nikon FE, FG, and Nikomat FT2. IMHO, Nikons are lovely cameras but overrated. Nikon lenses are great but so are Pentax and Minolta lenses. I'm pretty sure that if I posted photos of similar scenes taken with my Nikon FE/Nikkor 50/1.4 and my $7 Sears KS Auto/Sears 50/1.7, no one could which image was taken with which camera.
I'd keep the ME Super -- I have one and it's one of my favorite walk-around cameras. Nothing (with the possible exception of the Olympus OM-series) can touch it for light weight and easy handling.
I'd consider adding a longer lens to your collection -- the SMC Pentax-M 135/3.5 is very affordable, as is the Pentax 80-200 f/4.5 zoom, and both are compact and light. A 28mm is good to have as well -- Pentax's own 28s are expensive but I think the old Vivitar 28/2.8 does a great job.
I'd consider another body. The Pentax KX is a favorite of mine; it's all-manual, all-mechanical, and lots of fun to use. Also very affordable ($75-$100). I personally prefer it to the much-vaunted MX. Another great source of Pentax K-compatible bodies is Ricoh -- great underrated cameras. They're cheap and the Sears-branded versions are even cheaper. The Ricoh XR-1/Sears KS-1000 is an all-mechanical body with a feature set similar to the KX. The Ricoh XR-2/XR-2s/Sears KS Auto has a similar feature set with an electronic shutter and automatic mode (and is easier to use in manual mode than your ME Super). he Ricoh KR-10/Sears KSX is another great camera. All of these cameras can be found on eBay for less than $40 and often less than $20. Or $10. TThey have polycarb bodies and aren't as refined as the Pentax Ks but are still magnificent.
Here's the best part: You can most likely get a Pentax KX body, a Sears KS Auto body, a Pentax 80-200 *and* a Vivitar 28/2.8 all for less than the cost of a Nikon FM.
Aaron