I have both a DSLR and a NEX, so I might be able to help clarify a few things that can help you make a decision.
First, they aren't DSLRs...that's the first thing! The NEX cameras are compact mirrorless cameras with their own dedicated interchangeable lens lines (still small but growing), sharing the same APS-C sized sensors as DSLRs. The form factor can be an advantage, for the reasons you mention about portability, lightness, and convenience...they are, even with kit lenses, a good bit smaller than even the smallest DSLRs. However, they do have to make obvious sacrifices to achieve that size and portability - namely overall focus speed, ergonomics, and maximum fast control.
To address some of your specific issues/questions:
-will I end up spending $1000+ once I get the accessories (or will I be disappointed & end up buying a more expensive DLSR)
Depends on what you need from a camera. Will you be disappointed with image quality? Not likely - the NEX sensor is as good as the better DSLRs out there, as capable at high ISOs, and can shoot as well in daylight as low light. Will you spend $1000 or more in accessories? That's up to you - some do - probably some spend thousands with the NEX systems. The system is built for vast expansion and growth - but can just as well work as a body with kit lens and not need more. What types of shooting you want to do, how much experimentation you try, whether you need specialized lenses for certain types of shooting, if you get addicted to collecting lenses just because they're shiny and neat...only you can tell! Will you be disappointed with the ergonomics or controls? Probably not if you just do casual shooting, daylight shooting, travel and scenic shooting, and the occasional of something else. However, want to get into long lens work, wildlife shooting, frequent low light work, sports or action photography...then you might find yourself hobbled or unable to do it with the NEX.
-some reviews indicate the lenses seem "flimsy"
That's probably the one thing that's easiest to knock down - the lenses are built like high-end jewelry - all metal, shiny aluminum, beautiful finish, damped action...nothing about the NEX5 or its lenses are anything less than the finest camera builds since the Leicas of the 1950s. They're gorgeously built.
-speed - will I need to carry a monopod/tripod to capture low-light shots? I'm not expecting miracles, but I'd like to do things like get night pics of Epcot WS
No...actually, that's one of the fortes of these cameras. They excel at high ISO work, even with the average kit lenses that aren't that fast...and have some seriously nice tricks up their sleeves like automatic, self-aligning 6-image stacking allowing use of ISOs to 12800 with virtually no noise and excellent detail retention. These cameras are low light beasts. Invest in a conversion adapter and pair them with a good, fast prime lens, and they become one of the best cameras for low light handheld work INCLUDING most DSLRs.
-menu confusion - some reviews I've found say the menu is difficult and not intuiative
This was a problem when they came out. No question, the menus and controls were simplistic and unintuitive, clearly designed for folks intending to stick the camera in Auto mode and never look at a button again. I personally wouldn't have touched one of these cameras with a 10-foot pole when they came out, being a control freak. However, a few months back, Sony released their 2nd firmware for the camera, and this one specifically addressed the controls - the change was so entire that the camera went from one of the worst to shockingly good and intuitive, and one of the better choices for knowledgeable photographers who want to control their camera. There are 7 direct buttons on the camera body (5 of them incorporated into the multicontroller wheel). It looks like there'd be no way to directly access important controls. However, the firmware change activated 6 of those 7 buttons for direct, one-touch access to important parameters, with 2 of those buttons being user programmable to hold the function of your choice. The end result is that you can set up the camera to have direct control over ISO, WB, Focus mode, drive mode, EV, metering mode, flash control, focus area, metering area, or DRO/HDR - up to 6 at once. It's amazingly simple to manually control the camera.
How and what you shoot is going to be the big factor. If most of your shooting is scenic, landscape, travel, snapshots, portrait, or general photography, the NEX should be a solid camera with excellent performance in a compact package. If your shooting is going to be fast paced, heavy bursting, action, movement, tracking, wildlife, big zoom work, etc...then the NEX simply can't keep up and is the wrong tool for the job.
One side perk to mention for the NEX system that is a reason I picked one up as a second camera - the registration distance between lens and sensor is almost nil - without the mirror assembly, it's closer than any other mount. Which means with a simple cold adapter that spaces a lens the proper registration distance from the sensor, you can use literally any lens from any mount ever made on the NEX. There are dozens of adapters for every lens you can practically think of: Canon manual, EOS, rangefinder, Nikon EF, manual, rangefinder, Olympus OM, Pentax K and screw, Leica M, R, and screw, M42, Olympus Pen, and the list goes on. All of these lenses work 'old school' on the NEX - manual focus and aperture using the lens rings. Many of these lenses can be found dirt-cheap, and the adapters run $25 or less for most mounts...so picking up a fast prime is a cinch and doesn't have to break the bank.
I personally am a DSLR fan, and much of my shooting cannot be done without one. I could not shoot what I do using only the NEX system, nor the Micro 4:3 system. But as a second body, they are marvelous, and capability wise for regular shooting and especially low light, they're amazing. It depends on what most of your shooting involves, to determine whether a compact mirrorless camera can work as your primary big-sensor digital, or if a full DSLR body is needed to shoot the type of stuff you do now, or intend to do in the future.
PS - just to add re the previous poster's point - yes, Sony does make several levels of DSLRs which are faster, more advanced, and more capable, so there is somewhere to move up to. It is a different lens mount though, which can be shared with the NEX through an adapter as with any other lens. For me, Sony makes some killer DSLRs that were clearly the choice for my needs and shooting, and have supported my hobby, my habit, and paid for themselves through my semi-pro use of them, and have excellent lenses both in the Sony line and the Minolta line. It's a valid point to pick based on future needs too, but that still doesn't always mean you're going to pick a Canon or a Nikon.
