We use them (a college).
Pro - use less space, wiring, power.
Con - You can't throw away the computer after 4 years and keep the monitor (or vice versa).
As for access to the case, it depends on what model you pick - some are easier to get into than others. We currently use this, which is from a supplier that specialises in UK education, and while it comes with security screws because of the market it is being sold into, it is actually quite easy to get into and maintain besides that. An iMac or an iMac clone style machine will obviously be a lot more difficult.
If you're buying 1500 seats, it shouldn't be too hard to negotiate an on-site 3 year warranty as part of the deal or a very cheap extra? That's what we do.
The models we buy are based on a standard case with more or less standard (but sometimes half-height) expansion cards, motherboards, drives. I don't see those kind of AIOs as a problem in any environment, at the end of the day they're a standard computer and a standard monitor in a fancy case with the graphics and power wires for the monitor routed internally...
I've got one of these at home that's 6 years old - it did 4 years running XP, Office, etc in a college classroom with barely a problem and when we 'scrapped' it I took it home, cleaned out a LOT of dust and fluff, installed a bigger hard drive, the fastest model of pentium 4 that the motherboard would take (a place near me had a sale on these for pennies), and a Aero capable graphics card and put windows 7 on it. It runs like a champ, and is a decent media centre PC in one of my spare bedrooms. Got a second one for a friend's nephew and he loves it. These are pretty robust and reliable machines.
The other type are what I think of as the iMac clones - which are based on making the AIO as compact as possible, and are typically based on laptop components and construction techniques. These are much harder to maintain and may come at a cost premium over and above the other type of AIO (let alone conventional computer and monitor combos) but have advantages if desk space is at an extreme premium, or in areas where appearance might be as important as functionality and certainly more important than ease of maintenance (e.g. reception areas, CEO's desk, etc).
I wouldn't like to say that one or the other type are good or bad, just that they're aimed at slightly different needs and markets. If ease of maintenance is important to you, then the question is what category do the Lenovo machines you're looking at fall into. I think the processor and chipset it uses are desktop parts, which sounds good, and this review talks about being able to open it up to work on HDD and memory without too much trouble which is promising. It also has power supply inside the case instead of external which I always think is a good thing.
If it were my employer's money and I was buying that many desktops I'd ask for an eval unit, and I'd pop the case to see if we felt it was maintainable (in fact we always do this for each year's round of purchasing) but this looks like a reasonable start.
