I've never found them to be the least bit invasive. They do naturalize well, forming larger colonies over time, but they are completely manageable otherwise. Unlike something like scilla or bluebells (Hyacinthoides hispanica or non-scripta), which is impossible to be rid of once established.

Muscari & Scilla do naturalize very well in Ontario. I wouldn't call them invasive but certainly the right bulb if you're looking for large swaths of blue in your garden with very little effort. I only wish Bluebells would do the same here rather than simply persisting.

I would call Muscari invasive. I have been trying to dig it out for years now. Not only does each bulb divide but they go to seed and spring up all over. When the clumps are thick the foliage smothers anything delicate in it's path. I've had much less trouble with scilla. I certainly is a gorgeous color but IF I ever get rid of it, I'll never plant it again.

Grape Hyacinths are moderate to good spreaders in my garden, depending on soil & sun/shade conditions, but are certainly not invasive. I have a small stream of them in a raised bed with loose dryish soil and with some building shade that maintain okay, but do not spread much at all. Others are spreading well under deciduous shrubs, but have died off in only slightly wetter areas.

Scillas (I have mostly S. siberica Spring Beauty), when given dry conditions under a crabapple tree (in my generally wettish clay gardens) have spread very vigorously and have crowded out other small bulbs. I've started lifting clumps of them away from the Ipheions and planting them in less hospitable areas of the yard. They are doing surprisingly quite well in a wetter/lower area around some Aronia shrubs, but without the rapid spread.

BTW, It took at least 3-4 years for me to get a small patch of spanish bluebells going. Maybe the bluebells will take off this coming spring.

The only problem I see with the grape hyacinths is the shabby off season foliage. But even winter burned green is nice to see in the cold of zone 5.

I have been debating the very same thing for a while now. As a compromise, you might want to go with a sterile variety, so they will only be able to spread by bulb division and not by self-seeding. Muscari armeniacum 'Saffier' and 'Blue Spike' are supposed to be sterile as is a hybrid called 'Valerie Finnis.' I think Muscari latifolium may be sterile also but not positive. If I put in any it will most likely be one of these.

Seems like most spring bulbs do better (naturalize/spread more) when they get dry conditions during the summer. Leslie, your experience certainly seems to bear this out. Although I had heard Ipheion can really take over but maybe yours are not as happy being dry? And I am curious to know what conditions your Spanish bluebells are in. I really wanted to plant some of those with my spring bulbs but I am afraid too since every comment I have read about them here says that they tend to be invasive.

The ipheions are in dry shade under a crabapple tree. They have done well, as has the chionodoxa in the same bed. Both get plenty of light before the leaves come out on the tree and then get covered up by large hostas which start up much later. (The hostas get some supplemental water in summer by soaker hose.) Neither the ipheions nor the chionodoxa bulbs can keep up with the scillas, which make very thick clumps very quickly and crowd out the others. I am now restricting the scilla to the house side of the bed and the other two to the other side of the bed.

The spanish bluebells are on the edge of a south facing patio under the partial shade of some taller plantings. This area is quite dry over winter (being in a rather high part of a sloping yard), but there is some run-off from the containers on the patio being watered frequently during the warm season. They have done well enough, but unfortunately have a somewhat weak color and also bloom a bit later in the bulb season than I would like, coming on when most bulbish things are done here and spring perennials are taking over the scene. I much prefer for its color and form, the spring ephemeral, Virginia Bluebells (Mertensia virginica), a herbaceous perennial, which blooms just about the same time.

I could attest to the statement that Spanish Bluebells are fast naturalizers, but wouldn't call them invasive.
Yes, from initial 200 bulbs planted 3 or 4 years ago I just harvested roughly 500 to 600 bulbs (plus another 100 or so were given to friends last year), but they all were in a same general areas where they were originally planted.
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Main problem with them for me is that they bloom in mid-May and foliage persist till July.

A pre-emergent weedkiller in areas you didn't want them should prevent them from spreading all over everywhere, I should think.

Not that pre-emergent stopped the purslane ...

My grape hyacinths have never been invasive, though they do get thick as their bulbs divide.