Hi! I'm a transit enthusiast based in NYC and I would love to answer this!
Honestly, LIRR is a pretty great system. It doesn't have the stunning scenery of Metro-North or the constant breakdowns unique charms of NJ Transit, but it's an efficiently designed and operated network that makes Long Island so heckin navigable! And most of the issues it has are operational rather than infrastructural.
First, it's almost fully electrified, with the third-best electrification coverage in the country after the 100% electrified systems in Philadelphia and Denver. The M7 trains are some of my favorites in the country, actually. The only three non-electrified branches are Montauk, Greenport, and parts of Oyster Bay and Port Jefferson. And so you've got smooth rides with quick acceleration, which is more than most of the country can say, even the parts that do have trains at all.
And the coverage—oh my god the trains go everywhere, and they've got good connections to local buses too! (Hempstead Transit Center, I love you.) There's even pretty good interchanges between lines, for instance at Valley Stream, Lynbrook, Mineola, Hicksville, and of course Jamaica, meaning it's possible to make trips within Long Island rather than just going between LI and NYC. Admittedly this would be better if Montauk trains between Jamaica and Babylon consistently stopped at Mineola and Hicksville instead of always skipping them, but I'll take it.
I guess this is mostly a review of LI itself, but the scenic value of the system is kinda mixed. There's a cool viaduct on the Port Washington branch (and I appreciate that it's the Gatsby train), and there are some cool ocean views on the Suffolk branches and also the Long Beach branch. Plus the Oyster Bay branch has a nice forest. But most of Nassau is just kinda constant sprawl, so LIRR definitely places last on scenery among the NYC systems.
The biggest problems at the moment actually arose after Grand Central Madison opened. The split service patterns are complicated and confusing, and the current status quo is an awkward compromise where Grand Central doesn't have enough trains but the ones from Penn are overcrowded. And to make matters worse, they eliminated timed transfers at Jamaica! So now it's a hassle if the branch you're on doesn't serve the terminal you need.
Anyway, to do a shameless self-promotion, I've made some LIRR maps on this blog! Also, if you're curious about the potential future of the network, check out ETA's modernization proposal—admittedly, LIRR is the least affected of the three systems, but it's still a pretty cool report and I got to do the maps for it! :D
