Music software crank and blogger Chris Randall wrote a typically thoughtful post on what makes something a good instrument. He should know - his company makes really good effects plug-ins, many of which I own and use.
My take? "Pro" isn't what you use. It's how you use it.
The notion of "inspiration" is too subjective for me to consider, though. For me, it's easy to be briefly inspired on any new instrument, from the sheer novelty of the sound or interface. I've come up with many a part or song by noodling around on someone else's gear.
And what inspires you today may be what you're sick of tomorrow.
Good instruments make it easy to get at sufficient depth to encourage creativity without either losing you in details, menus, and parameters or restricting your choices.
Sadly, I no longer have any of the gear dating back to the Pants' early days in the suburban garages of our youth. However, I do have a few pieces of gear I've been using since the late 80s and many I've been using for nearly 10 years.
The good stuff is limited just enough, and frequently references old, tried, tested designs - the Nord Lead 2 we use live is basically a digital Prophet-5 with more voices (sadly, it doesn't do much better than the original for a display).
Back in ye olde dayes (a.k.a. the 80s) the Pants threw down big cash (at the time) for a Yamaha DX-7. It was considered one of the "Seven Deadly Synths" one had to have for a complete studio/record/band/whatever. It remains one of the best-selling synths of all time. New Order used it for their famous bass line on "Bizarre Love Triangle". I think Howard Jones used one for "What Is Love?" And its electric piano sounds were inescapable for the whole of the 80s.
I hated it.
We only ended up using it on a few tracks on "Life, Style". It was hard to program, combining a brand-new synthesis methodology (frequency modulation, a.k.a. FM) with one of the worst user interfaces ever to grace a popular keyboard. At the time, we (like almost everyone else) were unable to move much beyond the presets.
Parenthetically, it also schooled me on the dangers of beverages in the studio - I spilled a bottle of diet Pepsi Free into the key portion a week before a major gig. Ended up costing several hundred dollars to fix and I had to borrow a Korg Poly-800 to cover for it. Ouch.
A few years ago I traded a hot pink Ibanez Steve Vai Jem 777 for a DX-7IIFD, largely because I was interested in exploring alternate tunings, which the IIFD did (and was one of the only keyboards to do so). By then, I had learned enough to understand FM - but the interface still got in the way.
And no matter what, the DX series always felt like work. I never once sat down and said "ooh, this makes me want to write a song!"
The first synth I ever had was a Casio CZ-101 which my parents bought me for Christmas. I loved that thing and could program it 9 ways to Sunday. My brother sold it a few years ago after I asked him to take care of it for me (I guess I should have been more specific about what I meant by "take care of it for me!").
I still have horrible ancient recordings of that thing - seems like I couldn't touch it without starting to write something. Ironically, it was only slightly easier to program than the DX, and far more limited. And definitely not considered "pro".
But given a choice between the two today, I'd grab that CZ-101.