Cons to consider

Seating will be counter height, not table, which is less comfortable for most people

It's permanent

Your island loses functional storage and prep space and becomes mostly just for seating (stools get in the way)

It's probably substantially more expensive than buying a table

Requires a big space (that's probably at least 8'x4.5-5' island) which would require a 17' wide or so kitchen to work well.

My initial plans had a 5 or 6 seat kitchen island, decided it used up too much space and went with a 4 seat bar line up instead.

We considered doing an island table but didn't want it at counter height as we have small children. We initially drew up our island looking like this, but smaller:

We thought about this, but I realized that to walk around the island, I would have to walk around the table also, more steps. Now I can walk between the island and the table. Also, it would be permanently fixed, and not flexible in case we wanted to move things around. Before we scrapped the idea, we were thinking of the table portion being normal table height.

We considered it but eventually scrapped it for the reasons Annac stated above

Thank you everyone who has responded. We will still have a formal dining room but no nook area. Has anyone gone through with the kitchen island table combo has their primary dining area and regretted the decision.

Sure, we did this in a house (that we have since sold). I designed it and I loved it. We didn't have room to have the big kitchen we wanted PLUS a standalone table, but did have room to make the island bigger with seating. We also had a separate dining table. We really enjoyed this set up (and didn't use the dining table unless we had extra people visiting). We had no problem with the counter height, though my elderly father did not like it. Also, it was admittedly a huge pain for the period my son was in a high chair because there are no chairs that tall. We still had plenty of room for island storage and didn't need any more kitchen cabinets. In fact, if we wanted to fit a table in the room, we would needed to cut out some cabinets. This allowed us to maximize cabinets. We're currently designing another kitchen for our other condo, and also struggling to fit all of the cabinets I want plus a table. The island seating is the perfect solution. I also really liked sitting at that island while my husband cooked and vice versa. Much more casual and intimate than sitting at a kitchen table. Here's a picture from the mls listing (ignore the wide angle lens :)

We had a very small nook that was also a corridor between the garage, family room, kitchen, and sliding door to patio. It fit a two person cafe table nicely but not a four person table. We are a household of two people so we used the space from the nook to put in a 72" x 32" island with counter seating for two comfortably, 3 in a pinch. It works for us. Traffic flow is improved because we aren't dodging a table in the middle of the small nook. We also have a dining room but will only use it a few times per year.

Since you want to expand to 6-8 people I think it is fair to look at the traffic flow to see how much walking around you will need to do. Depending upon where the doorways are located it can improve traffic flow into the room (as it did in our case) but makes it a longer path if you have to walk around the seating area.

Initially considered but scrapped the idea because 1) we wanted some separation between kitchen and table 2) didn't want such a long barrier island in our U shaped kitchen 3) prefer round table for our family of 4

The house we just bought has a bi-level T shaped island/table combo and I hate it. It looks weird, and awkward. I plan to remodel it, and raise the table part to counter height, and make it one large island, still with seating.