I used to root and ROM back when Android needed help. As in back during the 2.1-early 4.0 days when phones only had literally 100MB of storage and you had to use things like CPU speed utilities or Apps2SD root plugins and VB Supercharger Scripts or more efficient kernels. Plus, back in those days customization was found only in special custom ROMs like CyanogenMod. I don't think even Samsung Themes even compared to the amount of customization I got from CyanogenMod 7.1 on my LG Optimus V.
Now, as for ads, I don't get any on my Thunderbolt. That's probably because the apps are so outdated that the servers they depended on for the ad bars are long gone! Javascript is turned off on all browsers so no ads, and Accuweather is so old it no longer displays ads, and the versions of Angry Birds and Cut the Rope were old paid versions so no ads there, and ColorNote, 12C Financial Calculator, and even Slacker Radio have no ads. All of them are from the 2.3 days, too. Basically every app I've ever used on Android I backed up the APKs during the root days. I always use the same apps, don't have any need for 'updated' versions or modern ones. The few modern ones I use are on tablets (walmart Pay, Kroger Pay, Little Caesar's, and Calm)
The last custom ROM I played with was when I used my Nexus 6. This was when Android 9 was new, and I was disappointed in the lack of customization offered, and that was the only 9.0 ROM available at the time. Lineage is nowhere close to CyanogenMod 7.1. Ain't been a ROM like it since.