I've done this twice. First time we rented a room on site at US/IoA and had one-hour early admission to Harry Potter (only the Islands of Adventure portion was open then)...my travel companions actually did stay over (Royal Pacific); altho I was on the room reservation, I returned to OKW to sleep and then went back to meet my friends the next morning, too. Cabbed over both mornings--it's an expensive cab ride ($50 or so one-way, as I recall, no matter how many people are in the cab). Second time was last August and we stayed at BCV, rented a car (from the Dolphin) and drove over. My DS spent the better part of all day/night there (we got the one-day two-park pass); he took a break around 3 or so to drive me back to BCV, hung out w/me for a bit and then he went back. It's about a 20-30-minute drive over, a bit less in non-traffic hours.
Parking is easy-peasy...very well organized...but quite far from parking lots to park entrances...I'd say about a mile. The drive can be easy or hard (we went super early so no traffic to speak of; coming back was tougher as traffic was heavier--did not go/come at all during rush hour, tho).
For me, I MUCH prefer WDW--both resorts and parks. I'm a huge fan of Harry Potter Land (in both parks, altho I like the one in IoA better), but not much else I really like at either US or IoA...I don't need more than 4-5 hours for a US/IoA outing (I do "need" to go to Harry Potter Land every once in a while; I do LOVE it)...so I will always and forever prefer to come back to WDW...
The Royal Pacific was nice; the walk over to the parks was very pleasant. When we went (both times) we didn't need early access (and FOTL doesn't work for Harry Potter)...neither time did we encounter "bad" lines. Both trips were in late August (one 3 years ago; one last year)...
I'm planning another such excursion in May of this year...we haven't decided how we'll do "the commute," although I know I will keep our Poly villa--I know I will return to the Poly as soon as I "finish" Harry Potter Land ... but my two friends may do a room over at US/IoA--one hasn't ever been to US/IoA and wants to "do" both parks more thoroughly than I do, and the other one likes those parks better than I do. But I know I'll come back to WDW, probably after only a half day or so at IoA/US. We're thinking we may try Uber, altho one of us doesn't mind driving (the other 2 of us do) so we may rent a car instead...really can't emphasize enough how easy the drive over is (going, as we do, pre-dawn to arrive at park opening), altho it IS a long walk to/from parking and they don't have trams like they do at the Disney parks...
For me, the biggest issue is getting me off WDW property. Once I'm there, I just don't want to leave. Only for Harry Potter am I willing to go!