The "mana/stamina regeneration" in description refers to not only natural regeneration over time, but also abilities/passives that restore mana/stamina (e.g. Looked Like It Hurt for Rogues and Restorative Veil for Rift Mage), as well as mana/stamina recovery from auto-attack of non-staff (Blade of Tidarion excluded) weapons.
Due to designer oversight, the natural mana/stamina regeneration mechanism between mages and warriors/rogues are different. For mages the regeneration amount is a fixed percentage of maximum mana (i.e. 6% per second by default), while for warriors and rogues, the amount is a fixed numeric one (i.e. 4 instead of 4% per second by default) in case of conflict between this and stamina recovery from auto-attacks.
As a result, the impact of mana/stamina regeneration penalty applied by Sigil of the Great Bear varies depending on classes and specializations. For mages, assume no other abilities/passives affecting natural mana regeneration enabled, natural mana regeneration amount is practically the same [100*6%=6 versus 200*6%*(1-50%)=6]. However, for Rift Mage and Necromancer, the effectiveness of their related mana recovery passives (namely Restorative Veil and Death Siphon) halved in the meantime.
For warriors and rogues, since the default natural stamina regeneration amount is a fixed numeric one, the character is actually worsened off by equipping Sigil of the Great Bear, where natural stamina regeneration amount per second decreases from 4 to 4*(1-50%)=2. Same scenario goes to stamina regeneration from auto-attacks and passives.
Nevertheless, the sigil still has its utility since a number of talent combos are non-achievable if maximum mana/stamina cannot be leveled up. Hence players may choose whether to equip Sigil of the Great Bear cautiously based on their needs in combat experiences. Generally speaking, mages especially Knight-Enchanters are mostly unaffected by the negative effect, while warriors and rogues might as well find better alternatives (e.g. Stamina Amulet) or gears that diluted the penalty (e.g. Amulet of Renewal).
The game caps mana and stamina at 200, therefore combining this sigil with other mana/stamina accessories will never surpass this cap. An unofficial fix for pcis available here.
The sigil applies 100% bonus only to base mana/stamina, ignoring bonuses.